Write cache enable or disable?

[JoeAtWork]

Hi All,

Is it good or a requirement to disable the on disk write cache? If I see 10-13% fluctuation in vdev usage I think this means the larger vdev will stall the scrub and slow down. This is my lab where the data is random dd 100gb files and then I copy them to fill the pool to 79%

I noticed this with 24 mirrors striped and when I did 6 vdevs of RAIDz2 I could really see the one vdev doing all the work on a scrub. It takes 6 hours or so to scrub this 79% zpool and I start to see where all the IO is going to the first vdev. I see the same thing when doing striped mirrors.

 

[Davvo]

Do you mean the ZIL?

Some insights into SLOG/ZIL with ZFS on FreeNAS

What is the ZIL? POSIX provides a facility for the system or an application to make sure that data requested to be written is actually committed to stable storage: a synchronous write request. Upon completion of a sync write request, the underlying filesystem is supposed to guarantee that a...

www.truenas.com

 

[HoneyBadger]

Hi All,

Is it good or a requirement to disable the on disk write cache?

No, provided that the write cache is "honest" and does not lie about its capabilities. ZFS is aware of and leverages the write cache on physical devices, issuing flushes where necessary to protect things like metadata (written synchronously) and uberblock updates.

ZFS should be correctly enabling the write cache on disks connected via HBAs or non-RAID SATA; the introduction of a RAID controller however will remove any guarantees of this. You can verify on a per-disk basis with camcontrol identify daX and looking/grepping for the "write cache" line, to ensure it shows as present and enabled.

 

[JoeAtWork]

Do you mean the ZIL?

Some insights into SLOG/ZIL with ZFS on FreeNAS

What is the ZIL? POSIX provides a facility for the system or an application to make sure that data requested to be written is actually committed to stable storage: a synchronous write request. Upon completion of a sync write request, the underlying filesystem is supposed to guarantee that a...

www.truenas.com

for i in {0..61}; do echo "WCE: 1" | camcontrol modepage da$i -m 0x08 -e; done