ZSTD Project Weekly Status Update

[Allan Jude]

This is the sixth weekly status report on the project to integrate ZSTD
into OpenZFS.

https://github.com/openzfs/zfs/pull/10631 - Improved the `zfs recv`
error handling when it receives an out-of-bounds property value.
Specifically, if a zfs send stream is created that supports a newer
compression or checksum type, the property will fail to be set on the
receiving system. This is fine, but `zfs recv` would abort() and create
a core file, rather than reporting the error, because it did not
understand the EINVAL being returned for that property. In the case
where the property is outside the accepted range, we now return the new
ZFS_ERR_BADPROP value, and the correct message is displayed to the user.
I opted not to use ERANGE because that is used for 'this property value
should not be used on a root pool'. The idea is to get this fix merged
into the 0.8.x branch for the next point release, to improve
compatibility with streams generated by OpenZFS 2.0


https://github.com/openzfs/zfs/pull/10632 - General improvement to error
handling when the error code is EZFS_UNKNOWN.


https://github.com/allanjude/zfs/commit/8f37c1ad8edaff20a550b3df07995dab80c06492
- ZFS replication compatibility improvements. As discussed on the
leadership call earlier this month, keep the compatibility simple. If
the -c flag is given, send blocks compressed with any compression
algorithm. The improved error handling will let the user know if their
system can't handle ZSTD.


https://github.com/allanjude/zfs/commit/0ffd80e281f79652973378599cd0332172f365bd
- per-dataset feature activation. This switches the ZSTD feature flag
from 'enabled' to 'active' as soon as the property is set, instead of
when the first block is written. This ensures that the pool can't be
imported on a system that does not support ZSTD that will cause the ZFS
cli tools to panic.


I will be working on adding some tests for the feature activation.

I've been looking at ways to add tests for the replication changes, but
it doesn't seem to be easy to test the results of a 'zfs recv' that does
not know about ZSTD (where the values are outside of the valid range for
the enum). If anyone has any ideas here, I'd be very interested.


On 2020-07-20 23:40, Allan Jude wrote:
> This is the fifth weekly status report on the project to integrate ZSTD
> into OpenZFS.
> 
> https://github.com/c0d3z3r0/zfs/pull/14/commits/9807c99169e5931a754bb0df68267ffa2f289474
> - Created a new test case to ensure that ZSTD compressed blocks survive
> replication with the -c flag. We wanted to make sure the on-disk
> compression header survived the trip.
> 
> https://github.com/c0d3z3r0/zfs/pull/14/commits/94bef464fc304e9d6f5850391e41720c3955af11
> - I split the zstd.c file into OS specific bits
> (module/os/{linux,freebsd}/zstd_os.c) and also split the .h file into
> zstd.h and zstd_impl.h. This was done so that FreeBSD can use the
> existing kmem_cache mechanism, while Linux can use the vmem_alloc pool
> created in the earlier versions of this patch. I significantly changed
> the FreeBSD implementation from my earlier work, to reuse the power of 2
> zio_data_buf_cache[]'s that already exist, only adding a few additional
> kmem_caches for large blocks with high compression levels. This should
> avoid creating as many unnecessary kmem caches.
> 
> https://github.com/c0d3z3r0/zfs/pull/14/commits/3d48243b77e6c8c3bf562c7a2315dd6cc571f28c
> - Lastly, in my testing I was seeing a lot of hits on the new
> compression failure kstat I added. This was caused by the ZFS "early
> abort" feature, where we give the compressor an output buffer that is
> smaller than the input, so it will fail if the block will not compress
> enough to be worth it. This helps avoid wasting CPU on uncompressible
> blocks. However, it seems the 'one-file' version of zstd we are using
> does not expose the ZSTD_ErrorCode enum. This needs to be investigated
> further to avoid issues if the value changes (although it is apparently
> stable after version 1.3.1).
> 
> I am still working on a solution for zfs send stream compatibility. I am
> leaning towards creating a new flag, --zstd, to enable ZSTD compressed
> output. If the -e or -c flag are used without the --zstd flag, and the
> dataset has the zstd feature active, the idea would be to emit a warning
> but send the blocks uncompressed, so that the stream remains compatible
> with older versions of ZFS. I will be discussing this on the OpenZFS
> Leadership call tomorrow, and am open to suggestions on how to best
> handle this.
> 
> 
> On 2020-07-14 22:26, Allan Jude wrote:
>> In my continuing effort to complete the integration of ZSTD into
>> OpenZFS, here is my fourth weekly status report:
>>
>> https://github.com/allanjude/zfs/commit/b0b1270d4e7835ecff413208301375e3de2a4153
>> - Create a new test case to make sure that the ZSTD header we write
>> along with the data is correct. Verify that the physical size of the
>> compressed data is less than the psize for the block pointer, and verify
>> that the level matches. It uses a random level between 1 and 19 and then
>> verifies with zdb that the block was compressed with that level.
>>
>> I am still working on a solution for setting the zstd feature flag to
>> 'active' as soon as it is set, rather than only once a block is born. As
>> well as fixing up compatibility around zfs send/recv with the embedded
>> block points flag.
>>
>> This project is sponsored by the FreeBSD Foundation.
>>
>>
> 


-- 
Allan Jude

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