Dear developers,
I have been using ext4 for quite some time and I have some ext4
filesystems that led me to some questions.
I have at least 2 "large" filesystems (with 2TB each, both almost
full) dating back from 2011 and 2010. I believe that I even converted
one of them from ext3 to ext4, but my memory is not that clear after
almost a decade. As such, they were created without some useful
features that are useful nowadays, like inline files. With that in
mind, here are some questions:
1 - I know that some features can be enabled with tune2fs, but, in
particular, inline files don't seem to be. I've seen some people
indicate that using debugfs, I can mark the superblock as having
support for it. Would that really work? I don't plan on booting old
kernels. One of those filesystems is running on an armel device that
is quite slow and I would really like to avoid copying all the files
to an external HD, recreating the filesystem and, then, copying back
the files to the system.
2 - Is there any way to get transparent compression with ext4? That
would really, really rock and is, perhaps, one of the features that
some users like me would greatly benefit from.
Thanks for any pointers,
Rogério Brito.
--
Rogério Brito : rbrito@{ime.usp.br,gmail.com} : GPG key 4096R/BCFCAAAA
http://cynic.cc/blog/ : github.com/rbrito : profiles.google.com/rbrito
DebianQA: http://qa.debian.org/developer.php?login=rbrito%40ime.usp.br
On Mon, Aug 17, 2020 at 03:54:29AM +0000, Rog?rio Brito wrote:
> Dear developers,
>
> I have been using ext4 for quite some time and I have some ext4
> filesystems that led me to some questions.
>
> I have at least 2 "large" filesystems (with 2TB each, both almost
> full) dating back from 2011 and 2010. I believe that I even converted
> one of them from ext3 to ext4, but my memory is not that clear after
> almost a decade. As such, they were created without some useful
> features that are useful nowadays, like inline files. With that in
> mind, here are some questions:
>
> 1 - I know that some features can be enabled with tune2fs, but, in
> particular, inline files don't seem to be. I've seen some people
> indicate that using debugfs, I can mark the superblock as having
> support for it. Would that really work? I don't plan on booting old
> kernels. One of those filesystems is running on an armel device that
> is quite slow and I would really like to avoid copying all the files
> to an external HD, recreating the filesystem and, then, copying back
> the files to the system.
Inline data is not enabled by default, because there are still a few
corner cases that will cause stress tests to fail. So it's not
something I'd encourage enabling; it doesn't make that much difference
unless your workload creates huge numbers of small (< ~100 bytes)
files. If you do create lots of small files, but then don't attempt
to append to them, or truncate them, etc., later., inline_data will
work, but it's likely that the presence or absence of inline_data
probably won't make that much difference to your application, unless
you're doing something super unusual.
> 2 - Is there any way to get transparent compression with ext4? That
> would really, really rock and is, perhaps, one of the features that
> some users like me would greatly benefit from.
No, transparent compression is not a feature ext4 has. There has been
some thinking about creating a read-only compression feature,
primarily to speed up reading from slow devices, but full transparent
compression where users might want to seek to the middle of a
transparently compressioned file, and then writing into the middle of
said file, and making sure this is reliable even in the face of a
crash in the middle of doing such an update, etc., is something that
would be a large amount of work, for relatively little benefit, and so
I'd be surprised if that were ever to be supported in ext4.
Read-only compressed files is something that might happen at some
point, but while we have a design sketched out for it, no one is
currently actively working on that feature.
Cheers,
- Ted