On Thu, 05 May 2011 11:06:09 +0300 Nikolay Kichukov <hijacker@xxxxxxxxx> wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > Hi, > It seems like he is reading directly from the raid device and not through the filesystem. So there are no filesystem > caches in this way. A block device is actually implemented a lot like a filesystem which contains a single file with a trivial mapping from file-block to device-block. In any case, reading from a block device most definitely does go through the page cache, and repeatedly reading from a device which is substantially smaller than memory will cause subsequent reads to come from the cache. NeilBrown > > Cheers, > - -Nik > > On 05/05/2011 07:06 AM, Roman Mamedov wrote: > > On Thu, 5 May 2011 00:08:59 +0100 > > Liam Kurmos <quantum.leaf@xxxxxxxxx> wrote: > > > >> in my tests i read 1GB and throw away the data. > >> dd if=/dev/md0 of=/dev/null bs=1M count=1000 > > > > If you have enough RAM for disk cache, on the second and further consecutive > > invocations of this you will be reading mostly from the cache, giving you an > > incorrect inflated result. So either don't forget to drop filesystem caches > > between runs, or just test read performance with "hdparm -t /dev/mdX" instead. > > > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.4.11 (GNU/Linux) > Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ > > iQEcBAEBAgAGBQJNwlpxAAoJEDFLYVOGGjgXHS0IAJAVFwR0Y0SD6G4CTaViqNCv > 32s/VCZWTZMzXiLOrY5xCGFiuqPurmGLy/+aW+HeEYShMndkZQ8H8ZHlSx5L3OoH > SQnWS7gP5hUn2w9qamhtFk9iPWpx18ZzVqN/k9WyAWhY4Ro20G8PWI3/T4Q3+zam > WYJ6KglllX+BuQYVhmhwB1KGVFhmpQXBXKWVrcGIB7vyGnM5K9fWLbxd6VvghZvd > qpfrmPO2WvuqpxCS+YcZTqEg7osbzNB+W/6DMJ7BpxyUcxIEyXwBwZSUxsZe3WWo > oTfj0XUaVc17TPfKMCyLfgm+K6f+IJfKky5e5mJyFCjesDhqFngkpilft9xNxq0= > =qsmJ > -----END PGP SIGNATURE----- -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
