An Slog (you wrote ZIL but you meant Slog as ZIL is onpool logging while
Slog is logging on a dedicated device) is not a write cache. It's a
logging feature when sync write is enabled and only read after a crash
on next bootup. CIFS does not use sync per default and for NFS (that
wants sync per default) you can and should disable when you use NFS as a
pure backup target. Your benchmarks clearly show that sync is not
enabled otherwise write performance with a large vdev would be more like
30-50 MB/s instead your 400-700 MB/s.
If you want to enable sync, you should look at Intel Optane as Slog as
this is far better than any other Flash based Slog.
ZFS use RAM as read and write cache. The default write cache is 10% of
RAM up to 4GB so the first option to improve write (and read)
performance is to add more RAM. A fast L2Arc (ex Intel Optane, size
5-max 10x RAM) can help if you cannot increase RAM or if you want a read
ahead functionality that you can enable on an L2Arc. Even write
performance is improved with a larger read cache as even writes need to
read metadata.
Beside that, I would not create a raid Zn vdev from 24 disks. I would
prefer 3 vdevs from 8 disks or at least two vdevs from 12 disks as pool
iops scale with number of vdevs.
Gea
@napp-it.org
Am 18.06.2018 um 08:50 schrieb priyadarshan:
On 18 Jun 2018, at 08:27, Oliver Weinmann <oliver.weinm...@icloud.com> wrote:
Hi,
we have a HGST4u60 SATA JBOD with 24 x 10TB disks. I just saw that back then
when we created the pool we only cared about disk space and so we created a
raidz2 pool with all 24disks in one vdev. I have the impression that this is
cool for disk space but is really bad for IO since this only provides the IO of
a single disk. We only use it for backups and cold CIFS data but I have the
impression that especially running a single VEEAM backup copy job really maxes
out the IO. In our case the VEEAM backup copy job reads and writes the data
from the storage. Now I wonder if it makes sense to restructure the Pool. I
have to admit that I don't have any other system with a lot of disk space so I
can't simply mirror the snapshots to another system and recreate the pool from
scratch.
Would adding two ZIL SSDs improve performance?
Any help is much appreciated.
Best Regards,
Oliver
Hi,
I would be interested to know as well.
Sometimes we have same issue: need for large space vs need to optmise for speed
(read, write, or both). We also are using, at the moment, 10TB disks, although
never do RAID-Z2 with more than 10 disks.
This page has some testing that was useful to us:
https://calomel.org/zfs_raid_speed_capacity.html
Section «Spinning platter hard drive raids» has your use case (although 4TB,
not 10TB):
24x 4TB, 12 striped mirrors, 45.2 TB, w=696MB/s , rw=144MB/s , r=898MB/s
24x 4TB, raidz (raid5), 86.4 TB, w=567MB/s , rw=198MB/s , r=1304MB/s
24x 4TB, raidz2 (raid6), 82.0 TB, w=434MB/s , rw=189MB/s , r=1063MB/s
24x 4TB, raidz3 (raid7), 78.1 TB, w=405MB/s , rw=180MB/s , r=1117MB/s
24x 4TB, striped raid0, 90.4 TB, w=692MB/s , rw=260MB/s , r=1377MB/s
Different adapters/disks will change the results, but I do not thing ratio will
change much.
It would be interesting to see how ZIL would affect that.
Priyadarshan
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