On Thu, 21 Nov 2013 20:54:54 Adam Goryachev wrote: > On a pure storage server, the CPU would normally have nothing to do, > except a little interrupt handling, it is just shuffling bytes around. > Of course, if you need RAID7.5 then you probably have a dedicated > storage server, so I don't see the problem with using the CPU to do all > the calculations. How would the pure storage server in question send data out to client systems? SMB? NFS? IMAP? All options will take some CPU power. There's a common myth that people should use hardware RAID to save CPU time, that's obviously wrong in the case of RAID-1 (just writing the data twice) and RAID-5 (XOR for parity). But as we get into the more complex forms of parity CPU performance could become an issue. Of course in terms of overall system performance the cost of doing a RAID rebuild in terms of disk seeks will probably exceed the CPU use. As an aside, are there plans to limit the RAID rebuild speed for BTRFS? On Thu, 21 Nov 2013 18:30:49 Stan Hoeppner wrote: > I suggest that anyone in the future needing fast random write IOPS is > going to move those workloads to SSD, which is steadily increasing in > capacity. And I suggest anyone building arrays with 10-20TB drives > isn't in need of fast random write IOPS. Traditionally SCSI/SAS disks have tended to be a lot smaller than IDE/SATA disks. Now Dell has just started offering 2TB SAS disks while the largest SATA disks that they sell (In Australia on PowerEdge T110 servers at least) are also 2TB. Presumably RAID recovery time was one factor that made manufacturers not bother with making larger SCSI/SAS disks in the past. When 20TB disks become available a user could choose to just use the first 10TB of the disk. Even in the days when 40G disks were big some people would use less than the full capacity of the disk for performance. Will people really be storing data that is so important that triple parity is needed on 20TB disks but be unable to afford enough disks to use only the first 10TB of each disk? Is this a use-case that is worth coding for? -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/ -- 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
