On 2/14/2012 5:38 AM, keld@xxxxxxxxxx wrote: > On Mon, Feb 13, 2012 at 09:49:06PM -0600, Stan Hoeppner wrote: >> On 2/13/2012 5:02 PM, keld@xxxxxxxxxx wrote: >> >>> And anyway, I think a 7 spindle raid10,f2 would be much faster than >>> a md linear array setup, both for small files and for largish >>> sequential files. But try it out and report to us what you find. >> >> The results of the target workload should be interesting, given the >> apparent 7 spindles of stripe width of mdraid10,f2, and only 3 effective >> spindles with the linear array of mirror pairs, an apparent 4 spindle >> deficit. >> >>> I would expect a linear md, and also most other MD raids would tend to perform better in >>> the almost empty state, as the files will be placed on the faster parts of the spindles. >> >> This is not the case with XFS. >> >>> raid10,f2 would have a more uniform performance as it gets filled, because read access to >>> files would still be to the faster parts of the spindles. >> >> This may be the case with EXTx, Reiser, etc, but not with XFS. >> >> XFS creates its allocation groups uniformly across the storage device. >> So assuming your filesystem contains more than a handful of directories, >> even a very young XFS will have directories and files stored from outer >> to inner tracks. > > Would not even XFS allocate lower AGs (on faster tracks) first? > >> This layout of AGs, and the way XFS makes use of them, is directly >> responsible for much of XFS' high performance. For example, a single >> file create operation on a full EXTx filesystem will exhibit a ~30ms >> combined seek delay with an average 3.5" SATA disk. With XFS it will be >> ~10ms. This is because with EXTx the directories are at the outer edge >> and the free space is on the far inner edge. With XFS the directory and >> free space area few tracks apart within the same allocation group. Once >> you seek the directory in the AG, the seek latency from there to the >> track with the free space may be less than 1ms. The seek distance >> principal here is the same for single disks and RAID. > > > Well, I was talking for a given FS, including XFS. As raid10,f2 limits the read access to the > faster halves of the spindles, reads will never go to the slower halves. > > On other raid types than raid10,far with regular use, AGs in use and data will be spread > randomly over the disks, including the slower inner tracks. Here raid10,far will only > use the outer tracks for reading, with some speed-up as a consequence. Maybe I simply don't understand this 'magic' of the f2 and far layouts. If you only read the "faster half" of a spindle, does this mean writes go to the slower half? If that's the case, how can you read data that's never been written? -- Stan -- 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
