On Wed, Nov 30, 2011 at 12:37:55AM +0800, Pádraig Brady wrote:
> On 11/29/2011 03:35 PM, Jan Kara wrote:
> > Someone already mentioned this earlier and I don't think I've seen a
> > response: Do you have a realistic usecase for this? I don't think I've ever
> > seen an application reading file backwards...
>
> tac, tail -n$large, ...
Indeed!
tac-4425 [000] 73358.419777: readahead: readahead-random(dev=0:16, ino=1548445, req=750+1, ra=750+1-0, async=0) = 1
tac-4425 [004] 73358.442030: readahead: readahead-backwards(dev=0:16, ino=1548445, req=748+2, ra=746+5-0, async=0) = 4
tac-4425 [004] 73358.443312: readahead: readahead-backwards(dev=0:16, ino=1548445, req=744+2, ra=726+25-0, async=0) = 20
tail-4369 [000] 72633.696307: readahead: readahead-random(dev=0:16, ino=1548450, req=750+1, ra=750+1-0, async=0) = 1
tail-4369 [004] 72634.042106: readahead: readahead-backwards(dev=0:16, ino=1548450, req=748+2, ra=746+5-0, async=0) = 4
tail-4369 [004] 72634.043231: readahead: readahead-backwards(dev=0:16, ino=1548450, req=744+2, ra=726+25-0, async=0) = 20
tail-4369 [004] 72634.176216: readahead: readahead-backwards(dev=0:16, ino=1548450, req=724+2, ra=626+125-0, async=0) = 100
However I see the readahead requests always be snapped to EOF.
So it's obvious the "snap to EOF" logic need some limiting based on
max readahead size.
Thanks,
Fengguang
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