Re: Do I have a hardware or a software problem?
| От | Rosser Schwarz |
|---|---|
| Тема | Re: Do I have a hardware or a software problem? |
| Дата | |
| Msg-id | CAFnxYwjVjR3zfWSHraiha_SGBLsZbEDeVS3rzZb8wV_-x3zLUg@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Do I have a hardware or a software problem? (Evgeny Shishkin <itparanoia@gmail.com>) |
| Список | pgsql-performance |
On Tue, Dec 11, 2012 at 5:17 PM, Evgeny Shishkin <itparanoia@gmail.com> wrote:
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.
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.
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