On Fri, Nov 11, 2022 at 07:15:07AM +0100, hw wrote:
There was no misdiagnosis. Have you ever had a failed SSD? They usually just disappear.
Actually, they don't; that's a somewhat unusual failure mode. I have had a couple of ssd failures, out of hundreds. (And I think mostly from a specific known-bad SSD design; I haven't had any at all in the past few years.) I've had way more dead hard drives, which is typical.
There was no "not normal" territory, either, unless maybe you consider ZFS cache as "not normal". In that case, I would argue that SSDs are well suited for such applications because they allow for lots of IOOPs and high data transfer rates, and a hard disk probably wouldn't have failed in place of the SSD because they don't wear out so quickly. Since SSDs are so well suited for such purposes, that can't be "not normal" territory for them. Perhaps they just need to be more resilient than they are.
You probably bought the wrong SSD. SSDs write in erase-block units, which are on the order of 1-4MB. If you're writing many many small blocks (as you would with a ZFS ZIL cache) there's significant write amplification. For that application you really need a fairly expensive write-optimized SSD, not a commodity (read-optimized) SSD. (And in fact, SSD is *not* ideal for this because the data is written sequentially and basically never read so low seek times aren't much benefit; NVRAM is better suited.) If you were using it for L2ARC cache then mostly that makes no sense for a backup server. Without more details it's really hard to say any more. Honestly, even with the known issues of using commidity SSD for SLOG I find it really hard to believe that your backups were doing enough async transactions for that to matter--far more likely is still that you simply got a bad copy, just like you can get a bad hd. Sometimes you get a bad part, that's life. Certainly not something to base a religion on.
Considering that, SSDs generally must be of really bad quality for that to happen, don't you think?
No, I think you're making unsubstantiated statements, and I'm mostly trying to get better information on the record for others who might be reading.