* why md request buffers will not across devices
@ 2006-01-18 7:52 Zhikun Wang
2006-01-18 23:58 ` Neil Brown
0 siblings, 1 reply; 2+ messages in thread
From: Zhikun Wang @ 2006-01-18 7:52 UTC (permalink / raw)
To: linux-raid
hi,
I have a silly question. Why md request buffers will not across devices? That means Why a bh will only locate in a single storage device? I guess maybe file system has aligned the bh? Who can tell me the exact reasons? Thanks a lot!
zkwang
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: why md request buffers will not across devices
2006-01-18 7:52 why md request buffers will not across devices Zhikun Wang
@ 2006-01-18 23:58 ` Neil Brown
0 siblings, 0 replies; 2+ messages in thread
From: Neil Brown @ 2006-01-18 23:58 UTC (permalink / raw)
To: Zhikun Wang; +Cc: linux-raid
On Wednesday January 18, raidwang@gmail.com wrote:
> hi,
> I have a silly question. Why md request buffers will not
> across devices? That means Why a bh will only locate in a single
> storage device? I guess maybe file system has aligned the bh? Who
> can tell me the exact reasons? Thanks a lot!
>
If you are talking 'bh' then you are talking '2.4'.
In 2.4. all requests match the 'blocksize' of the device (typically 1K
or 4K) and are aligned to that size. As the chunksize is always a
multiple of the block size a block will never cross a chunk boundary
and so never cross a device boundary.
2.6 is quite different and md sometimes needs to split a 'bio' request
to feed part to one device and part to another.
NeilBrown
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2006-01-18 7:52 why md request buffers will not across devices Zhikun Wang
2006-01-18 23:58 ` Neil Brown
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