Mirrored SSDs for ZIL/SLOG - safety, flushing, capacitors

[Bob Friesenhahn]

On Thu, 2 Oct 2014, javocado wrote:

> I'm setting up a SLOG to improve performance on my ZFS-based fileserver. I
> plan to use 2 mirrored SSD's to create the log device. Before I continue,
> I have some specific questions how to approach which hardware to use.
>
> Primarily, I'm concerned with what damage/corruption may occur when there's
> a power loss.

The damage/corruption which might occur due to power loss is primarily 
dependent on the drive firmware.  If the firmware does persist the 
write then it says it does, then it is not necessary to have a 
supercap and there is no loss of data.  However, peristing the write 
can be rather slow (and tends to reduce drive lifetime) so SSD 
firmware often claims to have written the data and defers the write 
for a while in the hope of having more data to write at once.  If the 
write is deferred, then data will be lost without a supercap.  To make 
matters worse, the drive might not write the data to FLASH in the 
order requested and so the data on the drive might not be usefully 
coherent when power returns.

A really poor drive will corrupt the data, and might even corrupt data 
not directly involved in the write requests because the erasure-block 
size is much larger than the data block size, and because the firmware 
might decide to move some data around in the background and presumed 
written data may be left in an inconsistent state.

> 2. should I only use SSDs with capacitors? does anyone recommend any? I'm
> seeing good things about the Intel s3500

The Intel S3700 would be a better choice for this application since it 
is optimized for writes.

> 4. flushing: does anyone know if (and which) SSDs are respecting and
> flushing data to media when told?

Likely few do.  Unless the vendor specification claims to do so, it is 
best to assume that they do not.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/