[zfs-discuss] L2ARC and SLOG on HW RAID with writeback cache

Gionatan Danti g.danti at assyoma.it
Fri Apr 27 15:18:15 EDT 2018

Hi all,
given a system with a proper, writeback-enabled RAID controller which 
can both create a classical array on some disks *and* passthrough some 
other disks, is it a good idea to place L2ARC and SLOG on the HW array?

My reasoning is:
- using CentOS I would like to avoid ZFS on root, so I need some sort of 
mirrored system partition (ie: MDRAID or HW array);
- an HW RAID1 mirror provide resiliency and very easy replacement of a 
failed disk (ie: disk replacement is completely transparent both for the 
OS and the L2ARC/SLOG);
- a writeback-enabled RAID card give very low write latency, which is 
ideal for SLOG;
- data stay on SLOG for a short time only and are read on emergency 
recovery only, meaning the probability of an unrecoverable read error is 
very low;
- by using SSD for the system disks, I can partition them for L2ARC use;
- any failed/uncorrect L2ARC read will be trasparently redirected to the 
main pool;
- the main pool has full ZFS protection against bit root and the likes.

On the other hand, by using a software RAID1 array for the OS partition 
only, presenting the other SSD partitions directly to ZFS, I can stripe 
the L2ARC partition for added cache space (which is the solution I am 
commonly adopting). Moreover, I trust MDRAID way more the HW blackboxes.

Any thoughts?

Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
GPG public key ID: FF5F32A8

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