| From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Bruno Wolff III <bruno(at)wolff(dot)to>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Greg Stark <gsstark(at)mit(dot)edu>, Russell Smith <mr-russ(at)pws(dot)com(dot)au>, josh(at)agliodbs(dot)com, Postgres Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Checkpoint cost, looks like it is WAL/CRC |
| Date: | 2005-07-07 04:29:19 |
| Message-ID: | 200507070429.j674TKT04852@candle.pha.pa.us |
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| Lists: | pgsql-hackers |
Tom Lane wrote:
> Bruno Wolff III <bruno(at)wolff(dot)to> writes:
> > Are you sure about that? That would probably be the normal case, but are
> > you promised that the hardware will write all of the sectors of a block
> > in order?
>
> I don't think you can possibly assume that. If the block crosses a
> cylinder boundary then it's certainly an unsafe assumption, and even
> within a cylinder (no seek required) I'm pretty sure that disk drives
> have understood "write the next sector that passes under the heads"
> for decades.
SCSI tagged queueing certainly allows 512-byte blocks to be reordered
during writes.
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
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