Re: Do I have a hardware or a software problem?

On Tue, Dec 11, 2012 at 5:17 PM, Evgeny Shishkin <itparanoia(at)gmail(dot)com>wrote:

> Actually most of low-end SSDs don't do write caching, they do not have
> enough ram for that.
>

AIUI, *all* SSDs do write-caching of a sort: writes are actually flushed to
the NAND media by erasing, and then overwriting the erased space, and
erasing is done in fixed-size blocks, usually much larger than a
filesystem's pages. The drive's controller accumulates writes in an
on-board cache until it has an "erase block"'s worth of them, which are
then flushed. From casual searching, a common erase block size is 256
kbytes, while filesystem-level pages are usually 4k.

Most low-end (and even many mid-range) SSDs, including Sandforce-based
drives, don't offer any form of protection (e.g., supercaps, as featured on
the Intel 320 and 710-series drives) for the data in that write cache,
however, which may be what you're thinking of. I wouldn't let one of those
anywhere near one of my servers, unless it was a completely disposable,
load-balanced slave, and probably not even then.

rls

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:wq