From: | Gavin Sherry <swm(at)linuxworld(dot)com(dot)au> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Curt Sampson <cjs(at)cynic(dot)net>, Greg Stark <gsstark(at)mit(dot)edu>, ITAGAKI Takahiro <itagaki(dot)takahiro(at)lab(dot)ntt(dot)co(dot)jp>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: [PATCHES] O_DIRECT for WAL writes |
Date: | 2005-06-23 04:11:58 |
Message-ID: | Pine.LNX.4.58.0506231410270.20908@linuxworld.com.au |
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Lists: | pgsql-hackers pgsql-patches |
On Thu, 23 Jun 2005, Tom Lane wrote:
> [ on the other point... ]
>
> Curt Sampson <cjs(at)cynic(dot)net> writes:
> > But is it really a problem? I somewhere got the impression that some
> > drives, on power failure, will be able to keep going for long enough to
> > write out the cache and park the heads anyway. If so, the drive is still
> > guaranteeing the write.
>
> If the drives worked that way, we'd not be seeing any problem, but we do
> see problems. Without having a whole lot of data to back it up, I would
> think that keeping the platter spinning is no problem (sheer rotational
> inertia) but seeking to a lot of new tracks to write randomly-positioned
> dirty sectors would require significant energy that just ain't there
> once the power drops. I seem to recall reading that the seek actuators
> eat the largest share of power in a running drive...
I've seen discussion about disks behaving this way. There's no magic:
they're battery backed.
Thanks,
Gavin
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