On 9/27/07, Dan Pritts <[EMAIL PROTECTED]> wrote: > So I've been of the opinion (not backed up by experimental data) that > a concatenation (what linux md driver calls LINEAR; similar effects can > be realized with LVM) of two RAID1's would be better for BackupPC than > a RAID10. > > My rationale for this is that in RAID10, the disks are generally > seeking to the same spot, unless you have a write that doesn't span > across multiple raid stripes. This certainly happens, but i suspect > most writes span multiple stripes. > > i guess this really depends on the RAID stripe size, bigger would be better.
Looking at my average file size on one of my backuppc servers, it appears to be about 50KB. With a typical stripe size being 64KB, that would seem to indicate that your average write will fit on one stripe, so that may hurt your argument. Additionally, if we look at the big picture where we are writing out a bunch of files, these are pretty much guaranteed to be scattered all over the disk with your typical filesystem. Even a fresh filesystem will scatter new files over the disk to help avoid fragmentation issues should you decide to grow the file later. Now throw in the number of backups you are doing and you end up with too many variables to consider before assuming that a linear array will outperform a striped array. For random reads all over the disk, the performance should be similar for small files but large file reads should be up to twice as fast. Throw in multiple readers and the difference will narrow. > > Stay away from RAID5 unless you have a good > > controller with a battery backed cache. > > Even then, performance won't be great, especially on random small writes > (look up the "RAID5 write hole" and "read-modify-write" to understand why). But wait, I thought you said that the average write under backuppc load would be larger than a stripe? So which is it? ;-) -Dave ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ BackupPC-users mailing list BackupPC-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/backuppc-users http://backuppc.sourceforge.net/
