Re: SSDs with Postgresql?

On 14/04/2011 5:40 PM, Henry C. wrote:

>> The server-grade SLC stuff with a supercap, I hope, not the scary
>> consumer-oriented MLC "pray you weren't writing anything during power-loss"
>> devices?
>
> That's what a UPS and genset are for. Who writes critical stuff to *any*
> drive without power backup?

Even a server with redundant PSUs on a UPS backed by generators can go
down hard and unexpectedly. I'd be extremely nervous unless I could
afford to lose data since the last backup, or unless I had a really
trustworthy replication setup going.

Of course, it's wise to have one or both of those conditions be true
anyway, because no redundant storage system will save you from file
system corruption caused by an OS bug, data corruption caused by a Pg
bug, or a "DELETE FROM critical_table;" by a careless superuser. So I
guess it doesn't cost you more than the risk of some downtime to use
potentially corruption-prone non-supercap MLC, and it's probably worth
it for the performance in your clustered environment.

All I meant with my post was to raise the concern that the OP needs to
be aware of the untrustworthy nature of even the low-end Intel SSDs.
They're usable, you just have to compensate for their deficiencies.

> You have a valid point about using SLC if that's what you need though.
> However, MLC works just fine provided you stick them into RAID1. In fact, we
> use a bunch of them in RAID0 on top of RAID1.

RAID won't help you if they all drop their caches if the power supply
throws a wobbly. That said, it's certainly good for the lifetime issues
especially if the units are upgraded or rotated out in phases.

--
Craig Ringer

Tech-related writing at http://soapyfrogs.blogspot.com/