On 03/04/2012 03:27 AM, Steven Haigh wrote:
On 4/03/2012 1:24 PM, Stan Hoeppner wrote:
On 3/3/2012 6:56 PM, Steven Haigh wrote:
On 4/03/2012 8:42 AM, Stan Hoeppner wrote:
[snip]
blockdev --setra 8192 /dev/sd[abcdefg]
Read-ahead is per file descriptor, and occurs at the filesystem level.
The read-ahead value used is that of the device immediately underlying
the filessytem. So don't bother setting these above.
Interesting - I didn't think that was the case for whole disk arrays -
but there you go... Learnt something else :)
[snip]
echo 4096> /sys/block/$i/queue/read_ahead_kb
Eliminate this line ^^^^
Any insight into why? I would have thought that this would help -
however I'm not quite sure as to the values - as this is much less than
one chunk... That also being said, wouldn't it be a good idea to have
*some* readahead?
You read the answer up above, and commented on it. Maybe you didn't
fully understand? Or maybe it's because you don't know that these two
are functionally equivalent?
blockdev --setra X
echo X> /sys/block/$i/queue/read_ahead_kb
Ahhh - you're spot on... I didn't think they had the same functionality!
Btw, /sys/block/$i/queue/read_ahead_kb is deprecated, as its not a
kernel-internal queue setting, but rather bdi (backing device) related.
So /sys/block/$i/bdi/read_ahead_kb is recommended to be used (I guess
queue/read_ahead_kb will go away sometime in the future). In
/sys/class/bdi you can even control read-head of non-block device file
systems.
Cheers,
Bernd
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