Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:document conditions when reliable operation is possible)

From: jim owens
Date: Mon Aug 31 2009 - 11:16:04 EST

Christoph Hellwig wrote:
On Mon, Aug 31, 2009 at 09:19:58AM -0400, Mark Lord wrote:
In my opinion even that is too weak. We know how to control the cache
settings on all common disks (that is scsi and ata), so we should always
disable the write cache unless we know that the whole stack (filesystem,
raid, volume managers) supports barriers. And even then we should make
sure the filesystems does actually use barriers everywhere that's needed
which failed at for years.

That stack does not know that my MD device has full battery backup,
so it bloody well better NOT prevent me from enabling the write caches.

No one is going to prevent you from doing it. That question is one of
sane defaults. And always safe, but slower if you have advanced
equipment is a much better default than usafe by default on most of
the install base.

I've always agreed with "be safe first" and have worked where
we always shut write cache off unless we knew it had battery.

But before we make disabling cache the default, this is the impact:

- users will see it as a performance regression

- trashy OS vendors who never disable cache will benchmark
better than "out of the box" linux.

Because as we all know, users don't read release notes.

Been there, done that, felt the pain.

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