Re: [reiserfs-list] Re: [PATCH] 2.4.10 improved reiserfs a lot, but could still be better

ę[@─ě ┐8■ ┐Justin T. Gibbs (pcg@goof.com)
Mon, 24 Sep 2001 18:40:08 +0200


On Mon, Sep 24, 2001 at 09:15:19AM -0700, Nicholas Knight <tegeran@home.com> wrote:
[turning off write-cache]
> I'm sorry, but that's not acceptable.

(I had it turned off for a long time, until I reasoned: real power-outages
are very rare, so I can leave it turned on anyways and risk a
filesystemcheck after outages).

The reason this kills performance ALWAYS is that ide does not support large
enough transfer sizes (8-32k on most drives) to fill one track.

Turning off write caching has a big chance of lowering your transaction
throughput to the drive's RPM. Combined with linux' not-that-optimal elevator
and write behaviour this has good chances of costing a lot of performance.

TCQ will obviously help, but I somehow doubt it will work fine - even with
SCSI TCQ is a nightmare (the aic7xxx drive regularly kills my system if
tagged queueing is enabled for example). IDE currently is a mess (I do
_not_ expect my drive performance to simply halve just because two devices
to share the bus, even if this is how conservative ide is destined to
work).

I am convinced that there is a way of creating a hard write barrier (e.g.
a cache flush that waits) with most if not all ide disks - putting them
into powersave should work, if nothing else ;)

So apart from driver issues (such as TCQ), the mid-layer needs to be improved
(and plans already exist) to support semi-ordered writes and give as much
control over the device cache as possible.

Not to mention that the VM needs improvements here as well.

I didn't say much more than Alan implied: we have to live with it, so we
better think about making it work.

-- 
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      ---==---(_)__  __ ____  __       Marc Lehmann      +--
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