From: | "Magnus Hagander" <magnus(at)hagander(dot)net> |
---|---|
To: | cmcdevitt(at)greenplum(dot)com |
Cc: | tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com, itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp, pgsql-patches(at)postgresql(dot)org |
Subject: | Re: O_DIRECT support for Windows |
Date: | 2007-01-17 07:51:58 |
Message-ID: | 20070117081442.D2015DCC02C@svr2.hagander.net |
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Lists: | pgsql-hackers pgsql-patches |
> People seem to be confusing sector size and cluster size.
>
> Microsoft Windows assumes sectors are 8k or less on hard drives (99% are
> 512 bytes).
Do you have any doc ref for this? I beleive you but I've been searching for docs on that and found nothing.
>
> Cluster size is the allocation unit. On windows, this can be 512 to
> 256k (max 64k with 512 byte sectors).
> NTFS (which I think we need) is limited to 64k, last I looked.
Correct.
> On RAID devices, the allocation unit might actually be larger, but
> usually the *sector* size of these devices is still 8k or less (usually,
> they mimic the 512 byte sector size, because too much software assumes
> this)
Usually being the thing that might require an extra check.
> And that restriction is only for certain drivers and devices. Many
> don't enforce the restriction.
> But to be safe, sector alignment is best, because there are some drivers
> that care.
exactly. so we need that check to be sure, don't we?
/Magnus
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