Re: Block Device Caching

[Andreas Dilger <adilger@clusterfs.com>]

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On Jun 30, 2004  00:20 +0200, Markus Schaber wrote:
> During our application testing, we noticed that our application (that
> operates directly on a LVM volume) we noticed that it seems the read
> data does not go into any cache.
> 
> Now we did some tests using dd blocksize=1M count=1000:
> 
> Using dd directly on the /dev/daten/testing lvm volume, we read about 95
> MBytes/Seconds. Issuing multiple dds in sequence gives little variance in IO
> speed (between 90 and 100 MB/sec).
> 
> When we create a file system on this volume, and mount it, and we create
> a 1G file there, the dd gives us the same 95 MB/sec on the first read
> after the mount, and approx. 480 MB/Sec on subsequent reads.
> 
> This lead us to the conclusion that block devices do not cache, but the
> filesystem does. But subsequently, I ran some tests on my developer
> machine (Pentium 4 Mobile Laptop).
> 
> When dd'ing 100MB from /dev/hda5, the first read gives about
> 22MBytes/Sek (which seems okay for a 2.5" IDE Disk), but subsequend
> reads give about 389MBytes/sek (which is impossible to achieve using
> such hardware). Interestingly, this happens on a mounted partition,
> while when unmounting the partition, caching does not work. But for the
> /dev/daten/testing above, mounting a filesystem on the partition does
> not help in caching.

When you close the block device it flushes the cache for that device (inode).
If you kept the device open in some way (e.g. "sleep 10000000 < /dev/hda5")
then it should start caching the data between dd runs.

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://members.shaw.ca/adilger/             http://members.shaw.ca/golinux/


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