Hello,
I put up a bug report for this issue at:
https://www.illumos.org/issues/5339
I also wanted to get some general feedback from other potentially
interested parties. The gist of the issue is as follows (more detail at
the bug report):
OS:
OmniOS v11 r151012 (omnios-10b9c79)
Hardware:
CPUs: dual Intel Xeon E5-2620v2 (hexa-core 2.1GHz)
RAM: 64GiB 1600MHz ECC Reg DDR3
SSDs: Samsung 845DC Pro 400GB connected to Intel C602J SATA3 (6Gbps)
HDDs: HGST HUS724040ALS640 (4TB SAS 7200 RPM) connected to LSI SAS 2308
via LSI SAS2 (6Gbps) expander backplane with 4x SAS2 controller links
network: not in the path for this testing, but Intel I350
The system is completely empty and unused besides this testing.
steps to reproduce:
1. create the simplest test pool with one vdev and one slog
(demonstrative purpose only):
zpool create -f testpool c1t5000CCA05C68D505d0 log c5t0d0
2. using your freshly created, empty, and inactive pool, create a
randomized file for testing:
openssl enc -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1
2>/dev/null | base64)" -nosalt < /dev/zero | dd iflag=fullblock
of=/testpool/randomfile.deleteme bs=1M count=4096
3. warm up the ARC for fast reads on your file:
dd if=/testpool/randomfile.deleteme of=/dev/null bs=1M
4. write 100MiB synchronously and observe slog "alloc" with 'zpool
iostat -v':
dd if=/testpool/randomfile.deleteme of=/testpool/newrandom.deleteme
bs=1M oflag=sync count=100 ; zpool iostat -v testpool 1 7
before running any additional tests, run:
rm -f /testpool/newrandom*
expected results:
slog 'alloc' peak at 100MiB
actual results
slog 'alloc' peak at 200MiB
To observe this, your slog has to be sufficiently fast to accept the
data within the time span of a single txg (5 seconds by default).
I've also observed the same behavior on the same hardware under FreeNAS
(essentially FreeBSD 9 with some backports from 10 and a NAS specific
web gui). The test had to be slightly different (openssl and dd a little
different there) by way of using the sync=always zfs property to force
sync writes and use of the slog, but the outcome was the same except for
about 20% better sync write performance under FreeNAS.
This leads me to believe that this is in the ZFS code and not an OS
peculiarity.
Is there a known reason why I'm seeing double writes to the slog? Am I
alone or are others also seeing the same data amplification for sync
writes with a slog?
Sincerely,
Andrew Kinney
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