2008-07-07 03:59:35

by Shehjar Tikoo

[permalink] [raw]
Subject: Doc for adding new NFS export option

Hi All

Is there a document that enumerates which files and functions I need
to add stuff to, for adding a new server-side export option?

I've tried adding it to some of the functions in the exportfs related
code in nfs-utils and also in the kernel's export related headers but
I dont think that is all there is to it.

I ask this because without my new option, I am able to mount the
export correctly but with it, the mount command fails with the
following message:

mount.nfs: 192.168.10.5:/data/nfstest failed, reason given by server:
Permission denied

I'd appreciate any help.

Thanks
Shehjar


2008-07-07 06:29:27

by NeilBrown

[permalink] [raw]
Subject: Re: Doc for adding new NFS export option

On Monday July 7, shehjart-YbfuJp6tym7X/[email protected] wrote:
> Hi All
>
> Is there a document that enumerates which files and functions I need
> to add stuff to, for adding a new server-side export option?

No. I think the theory is that unless you have read and understood
the code, you should really be thinking about adding new options.
And if you have, then you don't need any documentations.
:-)

>
> I've tried adding it to some of the functions in the exportfs related
> code in nfs-utils and also in the kernel's export related headers but
> I dont think that is all there is to it.

That sounds good, but without details, it is hard to be sure.
Don't be afraid to send a patch which shows what you are trying to do.

>
> I ask this because without my new option, I am able to mount the
> export correctly but with it, the mount command fails with the
> following message:
>
> mount.nfs: 192.168.10.5:/data/nfstest failed, reason given by server:
> Permission denied

So what exactly is this new export option that you want to add?

NeilBrown

2008-07-09 02:59:25

by Shehjar Tikoo

[permalink] [raw]
Subject: Re: Doc for adding new NFS export option

Neil Brown wrote:
> On Monday July 7, shehjart-YbfuJp6tym7X/[email protected] wrote:
>> Hi All
>>
>> Is there a document that enumerates which files and functions I need
>> to add stuff to, for adding a new server-side export option?
>
> No. I think the theory is that unless you have read and understood
> the code, you should really be thinking about adding new options.
> And if you have, then you don't need any documentations.
> :-)
>
>> I've tried adding it to some of the functions in the exportfs related
>> code in nfs-utils and also in the kernel's export related headers but
>> I dont think that is all there is to it.
>
> That sounds good, but without details, it is hard to be sure.
> Don't be afraid to send a patch which shows what you are trying to do.

See the two diffs attached here:

1. nfs_utils_add_prealloc.diff
Adds support for "prealloc" and "no_prealloc" export options to
nfs-utils. Based on the latest nfs-utils at:
git://linux-nfs.org/nfs-utils

2. add_nfsd_prealloc.diff
Adds support to kernel for the two options above. Based on 2.6.25.10
git checkout from:
git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-2.6.25.y.git

The patches dont actually make nfsd do anything at this point because
to do any "prealloc" related work, I first need to be able to mount
the export with the new option. :)

>> I ask this because without my new option, I am able to mount the
>> export correctly but with it, the mount command fails with the
>> following message:
>>
>> mount.nfs: 192.168.10.5:/data/nfstest failed, reason given by server:
>> Permission denied
The above behaviour was observed with nfs-utils 1.1.2.
The strange thing is, for nfs-utils from the git repo, with the
patches above, mount command returns the same error regardless of
whether "prealloc" option is specified in /etc/exports.

>
> So what exactly is this new export option that you want to add?

As the option's name suggests, the idea is to use fallocate support in
ext4 and XFS, to pre-allocate disk blocks. I feel this might help nfsd
sync writes where each write request has to go to disk almost ASAP.
Because NFSv3 writes have to be stable(..not sure about NFSv4..), the
write-to-disk and block allocation must happen immediately. It is
possible that the blocks being allocated for each NFS sync write are
not as contiguous as they could be for say, local buffered writes.
I am hoping that by using some form of adaptive pre-allocation we can
improve the contiguity of disk blocks for nfsd writes.

This will help in two ways:

1. Disk block allocation will be invoked in its entirety at less
frequent intervals(..i.e. not on every NFS write request..) because
we'll be pre-allocating larger blocks.
At this point, I am not sure what exactly the block allocation
overhead is, but I'll measure all that once I have a prototype working.

2. Read-ahead will benefit because the blocks being allocated are
contiguous.

Thanks
Shehjar

>
> NeilBrown


Attachments:
nfs_utils_add_prealloc.diff (2.22 kB)
add_nfsd_prealloc.diff (1.71 kB)
Download all attachments

2008-07-09 03:58:31

by NeilBrown

[permalink] [raw]
Subject: Re: Doc for adding new NFS export option

On Wednesday July 9, shehjart-YbfuJp6tym7X/[email protected] wrote:
> Neil Brown wrote:
> > On Monday July 7, shehjart-YbfuJp6tym7X/[email protected] wrote:
> >> Hi All
> >>
> >> Is there a document that enumerates which files and functions I need
> >> to add stuff to, for adding a new server-side export option?
> >
> > No. I think the theory is that unless you have read and understood
> > the code, you should really be thinking about adding new options.
> > And if you have, then you don't need any documentations.
> > :-)
> >
> >> I've tried adding it to some of the functions in the exportfs related
> >> code in nfs-utils and also in the kernel's export related headers but
> >> I dont think that is all there is to it.
> >
> > That sounds good, but without details, it is hard to be sure.
> > Don't be afraid to send a patch which shows what you are trying to do.
>
> See the two diffs attached here:
>
> 1. nfs_utils_add_prealloc.diff
> Adds support for "prealloc" and "no_prealloc" export options to
> nfs-utils. Based on the latest nfs-utils at:
> git://linux-nfs.org/nfs-utils
>
> 2. add_nfsd_prealloc.diff
> Adds support to kernel for the two options above. Based on 2.6.25.10
> git checkout from:
> git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-2.6.25.y.git
>
> The patches dont actually make nfsd do anything at this point because
> to do any "prealloc" related work, I first need to be able to mount
> the export with the new option. :)
>
> >> I ask this because without my new option, I am able to mount the
> >> export correctly but with it, the mount command fails with the
> >> following message:
> >>
> >> mount.nfs: 192.168.10.5:/data/nfstest failed, reason given by server:
> >> Permission denied
> The above behaviour was observed with nfs-utils 1.1.2.
> The strange thing is, for nfs-utils from the git repo, with the
> patches above, mount command returns the same error regardless of
> whether "prealloc" option is specified in /etc/exports.

You are restarting a newly compiled mountd I assume? I cannot think
what else might cause the problem.

>
> >
> > So what exactly is this new export option that you want to add?
>
> As the option's name suggests, the idea is to use fallocate support in
> ext4 and XFS, to pre-allocate disk blocks. I feel this might help nfsd
> sync writes where each write request has to go to disk almost ASAP.
> Because NFSv3 writes have to be stable(..not sure about NFSv4..), the
> write-to-disk and block allocation must happen immediately. It is
> possible that the blocks being allocated for each NFS sync write are
> not as contiguous as they could be for say, local buffered writes.
> I am hoping that by using some form of adaptive pre-allocation we can
> improve the contiguity of disk blocks for nfsd writes.
>

NFSv3 writes do not have to be stable. The client will usually
request DATA_UNSTABLE, and then send a COMMIT a while later. This
should give the filesystem time to do delayed allocation.
NFSv4 is much the same.
NFSv2 does require stable writes, but it should not be used by anyone
interested in good write performance on large files.

It isn't clear to me that this is something that should be an option
in /etc/exports.
When would a sysadmin want to turn it off? Or if a sysadmin did want
control, sure the level of control required would be the size of the
preallocation.

I would strongly suggest demonstrating that you can improve some
measure of performance using preallocation before you even begin to
think about having an export option to select it.

NeilBrown

2008-07-09 15:02:39

by [email protected]

[permalink] [raw]
Subject: Re: Doc for adding new NFS export option

On Wed, Jul 09, 2008 at 01:58:25PM +1000, Neil Brown wrote:
> NFSv3 writes do not have to be stable. The client will usually
> request DATA_UNSTABLE, and then send a COMMIT a while later. This
> should give the filesystem time to do delayed allocation.
> NFSv4 is much the same.
> NFSv2 does require stable writes, but it should not be used by anyone
> interested in good write performance on large files.
>
> It isn't clear to me that this is something that should be an option
> in /etc/exports.
> When would a sysadmin want to turn it off? Or if a sysadmin did want
> control, sure the level of control required would be the size of the
> preallocation.

An export option might be a useful for testing as a way to get quick
before vs after comparisons.

But, yeah, before merging it into mainline it'd be better if it was
something that could safely just be turned on all the time.

--b.

>
> I would strongly suggest demonstrating that you can improve some
> measure of performance using preallocation before you even begin to
> think about having an export option to select it.
>
> NeilBrown
> --
> To unsubscribe from this list: send the line "unsubscribe linux-nfs" in
> the body of a message to [email protected]
> More majordomo info at http://vger.kernel.org/majordomo-info.html

2008-07-09 22:44:57

by Shehjar Tikoo

[permalink] [raw]
Subject: Re: Doc for adding new NFS export option

Neil Brown wrote:
> On Wednesday July 9, shehjart-YbfuJp6tym7X/[email protected] wrote:
>> Neil Brown wrote:
>>> So what exactly is this new export option that you want to add?
>> As the option's name suggests, the idea is to use fallocate support in
>> ext4 and XFS, to pre-allocate disk blocks. I feel this might help nfsd
>> sync writes where each write request has to go to disk almost ASAP.
>> Because NFSv3 writes have to be stable(..not sure about NFSv4..), the
>> write-to-disk and block allocation must happen immediately. It is
>> possible that the blocks being allocated for each NFS sync write are
>> not as contiguous as they could be for say, local buffered writes.
>> I am hoping that by using some form of adaptive pre-allocation we can
>> improve the contiguity of disk blocks for nfsd writes.
>>
>
> NFSv3 writes do not have to be stable. The client will usually
> request DATA_UNSTABLE, and then send a COMMIT a while later. This
> should give the filesystem time to do delayed allocation.
> NFSv4 is much the same.
> NFSv2 does require stable writes, but it should not be used by anyone
> interested in good write performance on large files.
>
> It isn't clear to me that this is something that should be an option
> in /etc/exports.

For now, I only need this option so I dont have to rebuild the kernel
each time I want to toggle the "prealloc" option.

> When would a sysadmin want to turn it off? Or if a sysadmin did want
> control, sure the level of control required would be the size of the
> preallocation.

It might be a good idea to turn it off if the block allocation
algorithm slows things down when allocating large number of blocks.

True. If needed, we should be able to add entries in /proc that
control min, max and other limits on preallocation size.

> I would strongly suggest demonstrating that you can improve some
> measure of performance using preallocation before you even begin to
> think about having an export option to select it.

Agreed. I'll have some measurements soon after I have something working.

Thanks
Shehjar

2008-07-09 23:40:42

by Chuck Lever III

[permalink] [raw]
Subject: Re: Doc for adding new NFS export option

On Wed, Jul 9, 2008 at 6:30 PM, Shehjar Tikoo <shehjart-YbfuJp6tym7X/[email protected]> wrote:
> Neil Brown wrote:
>>
>> On Wednesday July 9, shehjart-YbfuJp6tym7X/[email protected] wrote:
>>>
>>> Neil Brown wrote:
>>>>
>>>> So what exactly is this new export option that you want to add?
>>>
>>> As the option's name suggests, the idea is to use fallocate support in
>>> ext4 and XFS, to pre-allocate disk blocks. I feel this might help nfsd sync
>>> writes where each write request has to go to disk almost ASAP. Because NFSv3
>>> writes have to be stable(..not sure about NFSv4..), the write-to-disk and
>>> block allocation must happen immediately. It is possible that the blocks
>>> being allocated for each NFS sync write are not as contiguous as they could
>>> be for say, local buffered writes.
>>> I am hoping that by using some form of adaptive pre-allocation we can
>>> improve the contiguity of disk blocks for nfsd writes.
>>>
>>
>> NFSv3 writes do not have to be stable. The client will usually
>> request DATA_UNSTABLE, and then send a COMMIT a while later. This
>> should give the filesystem time to do delayed allocation.
>> NFSv4 is much the same.
>> NFSv2 does require stable writes, but it should not be used by anyone
>> interested in good write performance on large files.
>>
>> It isn't clear to me that this is something that should be an option
>> in /etc/exports.
>
> For now, I only need this option so I dont have to rebuild the kernel each
> time I want to toggle the "prealloc" option.
>
>> When would a sysadmin want to turn it off? Or if a sysadmin did want
>> control, sure the level of control required would be the size of the
>> preallocation.
>
> It might be a good idea to turn it off if the block allocation algorithm
> slows things down when allocating large number of blocks.
>
> True. If needed, we should be able to add entries in /proc that control min,
> max and other limits on preallocation size.

Usually options specific to a particular physical file system are
handled with mount options on the server. NFS export options are used
to tune NFS-specific behavior.

Couldn't you specify a mount option that enables preallocation when
mounting the file system you want to export?

I can see having a file system callback for the NFS server that
provides a hint that "the client just extended this file and wrote a
bunch of data -- so preallocate blocks for the data, and I will commit
the data at some later point". Most file systems would make this a
no-op.

But I don't think this would help small synchronous writes... it would
improve block allocation for large writes.

--
Chuck Lever

2008-07-10 23:55:33

by Shehjar Tikoo

[permalink] [raw]
Subject: Re: Doc for adding new NFS export option

Chuck Lever wrote:
> On Wed, Jul 9, 2008 at 6:30 PM, Shehjar Tikoo <shehjart-YbfuJp6tym7X/[email protected]> wrote:
>> Neil Brown wrote:
>>> On Wednesday July 9, shehjart-YbfuJp6tym7X/[email protected] wrote:
>>>> Neil Brown wrote:
>>>>> So what exactly is this new export option that you want to add?
>>>> As the option's name suggests, the idea is to use fallocate support in
>>>> ext4 and XFS, to pre-allocate disk blocks. I feel this might help nfsd sync
>>>> writes where each write request has to go to disk almost ASAP. Because NFSv3
>>>> writes have to be stable(..not sure about NFSv4..), the write-to-disk and
>>>> block allocation must happen immediately. It is possible that the blocks
>>>> being allocated for each NFS sync write are not as contiguous as they could
>>>> be for say, local buffered writes.
>>>> I am hoping that by using some form of adaptive pre-allocation we can
>>>> improve the contiguity of disk blocks for nfsd writes.
>>>>
>>> NFSv3 writes do not have to be stable. The client will usually
>>> request DATA_UNSTABLE, and then send a COMMIT a while later. This
>>> should give the filesystem time to do delayed allocation.
>>> NFSv4 is much the same.
>>> NFSv2 does require stable writes, but it should not be used by anyone
>>> interested in good write performance on large files.
>>>
>>> It isn't clear to me that this is something that should be an option
>>> in /etc/exports.
>> For now, I only need this option so I dont have to rebuild the kernel each
>> time I want to toggle the "prealloc" option.
>>
>>> When would a sysadmin want to turn it off? Or if a sysadmin did want
>>> control, sure the level of control required would be the size of the
>>> preallocation.
>> It might be a good idea to turn it off if the block allocation algorithm
>> slows things down when allocating large number of blocks.
>>
>> True. If needed, we should be able to add entries in /proc that control min,
>> max and other limits on preallocation size.
>
> Usually options specific to a particular physical file system are
> handled with mount options on the server. NFS export options are used
> to tune NFS-specific behavior.
>
> Couldn't you specify a mount option that enables preallocation when
> mounting the file system you want to export?

Two points here:
For filesystems that support preallocation,
a) it is already enabled through the inode->i_ops->fallocate operation
while

b) leaving the decision about the size of the pre-allocation up to the
caller, in this case NFS, because the caller will know best about the
pattern of writes it is handing to the filesystem.

So yes, it'll need a NFS level parameter(s), be it an export option or
a module_param.
>
> I can see having a file system callback for the NFS server that
> provides a hint that "the client just extended this file and wrote a
> bunch of data -- so preallocate blocks for the data, and I will commit
> the data at some later point". Most file systems would make this a
> no-op.

Ideally, something like preallocation window should become part of the
VFS, like read-ahead data in struct file_ra_state, by adding a
relevant data structure to struct file but that is too big a change at
this point, considering the points below.

>
> But I don't think this would help small synchronous writes... it would
> improve block allocation for large writes.
>

In my really simple prototype, with 64k NFS wsize and a single client
writing a 2Gig file over software raid0, there is no improvement in
write performance for XFS, and a lower throughput figure for ext4 for
all pre-allocation sizes ranging from 5Megs to 100Megs. The read
throughput does improve slightly for ext4. Havent tested reads for XFS
yet. This is for 2.6.25.10. One reason for no performance gain in XFS
could be the fact that the disk in these tests was newly formatted and
the test file was the first file created on the new filesystem so the
blocks allocated in the "no_prealloc" case were mostly contiguous to
begin with(..although that is highly simplifying it..). Perhaps
running a test with multiple writer clients will give more
information. Regarding ext4, I have no idea yet as to why throughput
reduces on using pre-allocation, that too on a fresh filesystem.

I'll run a few more tests in the next few days, in the mean time,
would someone here like to take a look at the patch I have for this
and provide feedback?

Thanks
Shehjar

2008-07-11 04:31:54

by Chuck Lever III

[permalink] [raw]
Subject: Re: Doc for adding new NFS export option

On Thu, Jul 10, 2008 at 7:40 PM, Shehjar Tikoo <shehjart-YbfuJp6tym7X/[email protected]> wrote:
> Chuck Lever wrote:
>>
>> On Wed, Jul 9, 2008 at 6:30 PM, Shehjar Tikoo <shehjart-YbfuJp6tym7X/[email protected]>
>> wrote:
>>>
>>> Neil Brown wrote:
>>>>
>>>> On Wednesday July 9, shehjart-YbfuJp6tym7X/[email protected] wrote:
>>>>>
>>>>> Neil Brown wrote:
>>>>>>
>>>>>> So what exactly is this new export option that you want to add?
>>>>>
>>>>> As the option's name suggests, the idea is to use fallocate support in
>>>>> ext4 and XFS, to pre-allocate disk blocks. I feel this might help nfsd
>>>>> sync
>>>>> writes where each write request has to go to disk almost ASAP. Because
>>>>> NFSv3
>>>>> writes have to be stable(..not sure about NFSv4..), the write-to-disk
>>>>> and
>>>>> block allocation must happen immediately. It is possible that the
>>>>> blocks
>>>>> being allocated for each NFS sync write are not as contiguous as they
>>>>> could
>>>>> be for say, local buffered writes.
>>>>> I am hoping that by using some form of adaptive pre-allocation we can
>>>>> improve the contiguity of disk blocks for nfsd writes.
>>>>>
>>>> NFSv3 writes do not have to be stable. The client will usually
>>>> request DATA_UNSTABLE, and then send a COMMIT a while later. This
>>>> should give the filesystem time to do delayed allocation.
>>>> NFSv4 is much the same.
>>>> NFSv2 does require stable writes, but it should not be used by anyone
>>>> interested in good write performance on large files.
>>>>
>>>> It isn't clear to me that this is something that should be an option
>>>> in /etc/exports.
>>>
>>> For now, I only need this option so I dont have to rebuild the kernel
>>> each
>>> time I want to toggle the "prealloc" option.
>>>
>>>> When would a sysadmin want to turn it off? Or if a sysadmin did want
>>>> control, sure the level of control required would be the size of the
>>>> preallocation.
>>>
>>> It might be a good idea to turn it off if the block allocation algorithm
>>> slows things down when allocating large number of blocks.
>>>
>>> True. If needed, we should be able to add entries in /proc that control
>>> min,
>>> max and other limits on preallocation size.
>>
>> Usually options specific to a particular physical file system are
>> handled with mount options on the server. NFS export options are used
>> to tune NFS-specific behavior.
>>
>> Couldn't you specify a mount option that enables preallocation when
>> mounting the file system you want to export?
>
> Two points here:
> For filesystems that support preallocation,
> a) it is already enabled through the inode->i_ops->fallocate operation while
>
> b) leaving the decision about the size of the pre-allocation up to the
> caller, in this case NFS, because the caller will know best about the
> pattern of writes it is handing to the filesystem.
>
> So yes, it'll need a NFS level parameter(s), be it an export option or a
> module_param.

Oh, I see. "do" or "don't" have NFSD use the i_op->fallocate call on
file systems that support it.

>> I can see having a file system callback for the NFS server that
>> provides a hint that "the client just extended this file and wrote a
>> bunch of data -- so preallocate blocks for the data, and I will commit
>> the data at some later point". Most file systems would make this a
>> no-op.
>
> Ideally, something like preallocation window should become part of the VFS,
> like read-ahead data in struct file_ra_state, by adding a relevant data
> structure to struct file but that is too big a change at this point,
> considering the points below.

The VFS would be the right place for local file accesses.

I think it is reasonable for NFSD to also have some specific control
over this (apart from anything the VFS might do), for example by
invoking the i_op itself as needed -- NFS file accesses are different
from local accesses in subtle ways.

>> But I don't think this would help small synchronous writes... it would
>> improve block allocation for large writes.
>>
>
> In my really simple prototype, with 64k NFS wsize and a single client
> writing a 2Gig file over software raid0, there is no improvement in write
> performance for XFS, and a lower throughput figure for ext4 for all
> pre-allocation sizes ranging from 5Megs to 100Megs.

I would expect you might find differences depending on exactly when
the pre-allocation is performed, and how much data is coming over in
every WRITE request. If you try a megabyte at a time and a much
larger file, you might see greater differences.

Do you think it is useful to adjust your pre-allocation algorithm
based on the characteristics of the underlying metadevices, or do the
file systems handle that reasonably well for themselves?

Would you try a pre-allocation if the client extends a file with a
SETATTR before it pushes any data to the server?

> The read throughput does
> improve slightly for ext4. Havent tested reads for XFS yet.

There is a tool floating around called "seekwatcher" that can analyze
the seek activity on your server's file system. If you can measure a
reduction in seek activity during reads after writing a pre-allocated
file, I think that would be a success even if there wasn't a
measurable performance improvement during the read.

> This is for
> 2.6.25.10. One reason for no performance gain in XFS could be the fact that
> the disk in these tests was newly formatted and the test file was the first
> file created on the new filesystem so the blocks allocated in the
> "no_prealloc" case were mostly contiguous to begin with(..although that is
> highly simplifying it..). Perhaps running a test with multiple writer
> clients will give more information.

You might also consider some file system aging techniques to boost the
amount of fragmentation before running your test. Multiple concurrent
kernel builds, iozone runs, and keeping the size of your partition on
the small side are some ways to increase fragmentation.

> I'll run a few more tests in the next few days, in the mean time, would
> someone here like to take a look at the patch I have for this and provide
> feedback?

I think it would be OK to post such a patch to the list.

--
Chuck Lever

2008-07-11 12:28:43

by Shehjar Tikoo

[permalink] [raw]
Subject: Re: Doc for adding new NFS export option

Chuck Lever wrote:
> On Thu, Jul 10, 2008 at 7:40 PM, Shehjar Tikoo <shehjart-YbfuJp6tym7X/[email protected]> wrote:
>> Chuck Lever wrote:
>>> On Wed, Jul 9, 2008 at 6:30 PM, Shehjar Tikoo <shehjart-YbfuJp6tym7X/[email protected]>
>>> wrote:
>>>> Neil Brown wrote:
>>>>> On Wednesday July 9, shehjart-YbfuJp6tym7X/[email protected] wrote:
>>>>>> Neil Brown wrote:
>>>>>>> So what exactly is this new export option that you want to add?
>>>>>> As the option's name suggests, the idea is to use fallocate support in
>>>>>> ext4 and XFS, to pre-allocate disk blocks. I feel this might help nfsd
>>>>>> sync
>>>>>> writes where each write request has to go to disk almost ASAP. Because
>>>>>> NFSv3
>>>>>> writes have to be stable(..not sure about NFSv4..), the write-to-disk
>>>>>> and
>>>>>> block allocation must happen immediately. It is possible that the
>>>>>> blocks
>>>>>> being allocated for each NFS sync write are not as contiguous as they
>>>>>> could
>>>>>> be for say, local buffered writes.
>>>>>> I am hoping that by using some form of adaptive pre-allocation we can
>>>>>> improve the contiguity of disk blocks for nfsd writes.
>>>>>>
>>>>> NFSv3 writes do not have to be stable. The client will usually
>>>>> request DATA_UNSTABLE, and then send a COMMIT a while later. This
>>>>> should give the filesystem time to do delayed allocation.
>>>>> NFSv4 is much the same.
>>>>> NFSv2 does require stable writes, but it should not be used by anyone
>>>>> interested in good write performance on large files.
>>>>>
>>>>> It isn't clear to me that this is something that should be an option
>>>>> in /etc/exports.
>>>> For now, I only need this option so I dont have to rebuild the kernel
>>>> each
>>>> time I want to toggle the "prealloc" option.
>>>>
>>>>> When would a sysadmin want to turn it off? Or if a sysadmin did want
>>>>> control, sure the level of control required would be the size of the
>>>>> preallocation.
>>>> It might be a good idea to turn it off if the block allocation algorithm
>>>> slows things down when allocating large number of blocks.
>>>>
>>>> True. If needed, we should be able to add entries in /proc that control
>>>> min,
>>>> max and other limits on preallocation size.
>>> Usually options specific to a particular physical file system are
>>> handled with mount options on the server. NFS export options are used
>>> to tune NFS-specific behavior.
>>>
>>> Couldn't you specify a mount option that enables preallocation when
>>> mounting the file system you want to export?
>> Two points here:
>> For filesystems that support preallocation,
>> a) it is already enabled through the inode->i_ops->fallocate operation while
>>
>> b) leaving the decision about the size of the pre-allocation up to the
>> caller, in this case NFS, because the caller will know best about the
>> pattern of writes it is handing to the filesystem.
>>
>> So yes, it'll need a NFS level parameter(s), be it an export option or a
>> module_param.
>
> Oh, I see. "do" or "don't" have NFSD use the i_op->fallocate call on
> file systems that support it.
>
>>> I can see having a file system callback for the NFS server that
>>> provides a hint that "the client just extended this file and wrote a
>>> bunch of data -- so preallocate blocks for the data, and I will commit
>>> the data at some later point". Most file systems would make this a
>>> no-op.
>> Ideally, something like preallocation window should become part of the VFS,
>> like read-ahead data in struct file_ra_state, by adding a relevant data
>> structure to struct file but that is too big a change at this point,
>> considering the points below.
>
> The VFS would be the right place for local file accesses.
>
> I think it is reasonable for NFSD to also have some specific control
> over this (apart from anything the VFS might do), for example by
> invoking the i_op itself as needed -- NFS file accesses are different
> from local accesses in subtle ways.
>
>>> But I don't think this would help small synchronous writes... it would
>>> improve block allocation for large writes.
>>>
>> In my really simple prototype, with 64k NFS wsize and a single client
>> writing a 2Gig file over software raid0, there is no improvement in write
>> performance for XFS, and a lower throughput figure for ext4 for all
>> pre-allocation sizes ranging from 5Megs to 100Megs.
>
> I would expect you might find differences depending on exactly when
> the pre-allocation is performed, and how much data is coming over in
> every WRITE request. If you try a megabyte at a time and a much
> larger file, you might see greater differences.
>

Ok. It is possible that the sweet spot for ext4 preallocation
performance is even below 5Megs.

> Do you think it is useful to adjust your pre-allocation algorithm
> based on the characteristics of the underlying metadevices, or do the
> file systems handle that reasonably well for themselves?
>

It will be useful for filesystems like ext[34] that do not already
incorporate some form of volume management capabilities. For instance,
I am not fully aware of how XFS works but looking at some recent perf
measurements and the paper on XFS(Sweeney96) which does mention XFS
volume manager, one can guess that it exploits knowledge of underlying
arrangement of disks for higher performance. So in the case of XFS, I
doubt if there is much more we can do at NFS layer without interfering
with XFS behaviour. Thats a different story for ext[34]. The question
here is: How do I determine if we have a single disk or multiple disks
associated with the device nfsd is writing to?
It might be possible to make the preallocated blocks be distributed
over multiple devices to improve parallelism.
(...I can hear shouts of, "layer violation!" at the back. :) ....)


> Would you try a pre-allocation if the client extends a file with a
> SETATTR before it pushes any data to the server?

Definitely. In fs/nfsd/vfs.c:nfsd_setattr(..), this can be
special-cased to check if the file is getting extended and then use
preallocation if supported by the underlying filesystem.


Thanks for other tips.
Shehjar

>
>> The read throughput does
>> improve slightly for ext4. Havent tested reads for XFS yet.
>
> There is a tool floating around called "seekwatcher" that can analyze
> the seek activity on your server's file system. If you can measure a
> reduction in seek activity during reads after writing a pre-allocated
> file, I think that would be a success even if there wasn't a
> measurable performance improvement during the read.
>
>> This is for
>> 2.6.25.10. One reason for no performance gain in XFS could be the fact that
>> the disk in these tests was newly formatted and the test file was the first
>> file created on the new filesystem so the blocks allocated in the
>> "no_prealloc" case were mostly contiguous to begin with(..although that is
>> highly simplifying it..). Perhaps running a test with multiple writer
>> clients will give more information.
>
> You might also consider some file system aging techniques to boost the
> amount of fragmentation before running your test. Multiple concurrent
> kernel builds, iozone runs, and keeping the size of your partition on
> the small side are some ways to increase fragmentation.
>
>> I'll run a few more tests in the next few days, in the mean time, would
>> someone here like to take a look at the patch I have for this and provide
>> feedback?
>
> I think it would be OK to post such a patch to the list.
>