Re: mdadm raid1 read performance
From: NeilBrown <hidden>
Date: 2011-05-05 09:30:39
On Thu, 05 May 2011 11:06:09 +0300 Nikolay Kichukov [off-list ref] 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:quoted
On Thu, 5 May 2011 00:08:59 +0100 Liam Kurmos [off-list ref] wrote:quoted
in my tests i read 1GB and throw away the data. dd if=/dev/md0 of=/dev/null bs=1M count=1000If 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-----