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From:   Damien Le Moal <Damien.LeMoal@wdc.com>
To:     Randy Dunlap <rdunlap@infradead.org>,
        Niklas Cassel <Niklas.Cassel@wdc.com>,
        Jens Axboe <axboe@kernel.dk>
CC:     Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] null_blk: add support for max open/active zone limit
 for zoned devices
Thread-Topic: [PATCH v2] null_blk: add support for max open/active zone limit
 for zoned devices
Thread-Index: AQHWfHkH+20N/yg19E+BQ9ptEBewKg==
Date:   Thu, 27 Aug 2020 15:04:36 +0000
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On 2020/08/27 23:51, Randy Dunlap wrote:=0A=
> On 8/27/20 6:50 AM, Niklas Cassel wrote:=0A=
>> Add support for user space to set a max open zone and a max active zone=
=0A=
>> limit via configfs. By default, the default values are 0 =3D=3D no limit=
.=0A=
> =0A=
> Hi,=0A=
> =0A=
> How does a user find out about how to use/set these limits?=0A=
=0A=
For the setting part, this is for testing. So any value, even extreme ones =
(e.g.=0A=
1) would be OK to check that a software correctly handles write accesses to=
=0A=
zones for a device that has open/active zone limitations. A more practical =
way=0A=
is to reuse values of real devices. For instance, some SMR disks I use have=
 a=0A=
max open limit of 128 and max active 0 (there is no limit for active zones =
on=0A=
SMR disks as ZBC/ZAC specifications do not define this concept).=0A=
=0A=
Another example is our soon to come work on btrfs zone support which shows =
that=0A=
at the very least 6 active zones are needed. So tests can be performed with=
 that=0A=
minimum to check the file system and that its block allocator does not go=
=0A=
opening/activating too many zones.=0A=
=0A=
For the using part, the above btrfs example is good: if the FS tries to all=
ocate=0A=
blocks in too many inactive zones at the same time without first filling ou=
t=0A=
zones already active, it may exceed the limit and writes will fail. The FS =
must=0A=
thus be aware of the limits and its block al;locator tuned to limit block=
=0A=
allocations within a set of zones smaller than the maximum active limit.=0A=
=0A=
Does this answer your question ?=0A=
=0A=
> =0A=
> =0A=
>> Call the block layer API functions used for exposing the configured=0A=
>> limits to sysfs.=0A=
>>=0A=
>> Add accounting in null_blk_zoned so that these new limits are respected.=
=0A=
>> Performing an operating that would exceed these limits results in a=0A=
>> standard I/O error.=0A=
>>=0A=
>> A max open zone limit exists in the ZBC standard.=0A=
>> While null_blk_zoned is used to test the Zoned Block Device model in=0A=
>> Linux, when it comes to differences between ZBC and ZNS, null_blk_zoned=
=0A=
>> mostly follows ZBC.=0A=
>>=0A=
>> Therefore, implement the manage open zone resources function from ZBC,=
=0A=
>> but additionally add support for max active zones.=0A=
>> This enables user space not only to test against a device with an open=
=0A=
>> zone limit, but also to test against a device with an active zone limit.=
=0A=
>>=0A=
>> Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>=0A=
>> ---=0A=
>> Changes since v1:=0A=
>> -Fixed review comments by Damien Le Moal.=0A=
>>=0A=
>>  drivers/block/null_blk.h       |   5 +=0A=
>>  drivers/block/null_blk_main.c  |  16 +-=0A=
>>  drivers/block/null_blk_zoned.c | 319 +++++++++++++++++++++++++++------=
=0A=
>>  3 files changed, 282 insertions(+), 58 deletions(-)=0A=
> =0A=
> thanks.=0A=
> =0A=
=0A=
=0A=
-- =0A=
Damien Le Moal=0A=
Western Digital Research=0A=