Re: wishful thinking about atomic, multi-sector or full MD stripe width, writes in storage

From: Krzysztof Halasa
Date: Thu Sep 03 2009 - 19:51:10 EST

Ric Wheeler <rwheeler@xxxxxxxxxx> writes:

> The whole thread above is about software MD using commodity drives
> (S-ATA or SAS) without battery backed write cache.

Yes. However, you mentioned external RAID arrays disable disk caches.
That's why I asked if they are using SATA or SCSI/etc. disks, and if
they have battery-backed cache.

> Also, when you enable the write cache (MD or not) you are buffering
> multiple MB's of data that can go away on power loss. Far greater
> (10x) the exposure that the partial RAID rewrite case worries about.

The cache is flushed with working barriers. I guess it should be
superior to disabled WB cache, in both performance and expected disk
Krzysztof Halasa
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