On Sat, 30 Apr 2011 16:27:48 +0100 pg_xf2@xxxxxxxxxxxxxxxxxx (Peter Grandi) wrote: > While I agree with BAARF.com arguments fully, I sometimes have > to deal with legacy systems with wide RAID6 sets (for example 16 > drives, quite revolting) which have op-journaled filesystems on > them like XFS or JFS (sometimes block-journaled ext[34], but I > am not that interested in them for this). > > Sometimes (but fortunately not that recently) I have had to deal > with small-file filesystems setup on wide-stripe RAID6 setup by > morons who don't understand the difference between a database > and a filesystem (and I have strong doubts that RAID6 is > remotely appropriate to databases). > > So I'd like to figure out how much effort I should invest in > undoing cases of the above, that is how badly they are likely to > be and degrade over time (usually very badly). > > First a couple of question purely about RAID, but indirectly > relevant to op-journaled filesystems: > > * Can Linux MD do "abbreviated" read-modify-write RAID6 > updates like for RAID5? That is where not the whole stripe > is read in, modified and written, but just the block to be > updated and the parity wblocks. No. (patches welcome). > > * When reading or writing part of RAID[456] stripe for example > smaller than a sector, what is the minimum unit of transfer > with Linux MD? The full stripe, the chunk containing the > sector, or just the sector containing the bytes to be > written or updated (and potentially the parity sectors)? I > would expect reads to always read just the sector, but not > so sure about writing. 1 "PAGE" - normally 4K. > > * What about popular HW RAID host adapter (e.g. LSI, Adaptec, > Areca, 3ware), where is the documentation if any on how they > behave in these cases? > > Regardless, op-journaled file system designs like JFS and XFS > write small records (way below a stripe set size, and usually > way below a chunk size) to the journal when they queue > operations, even if sometimes depending on design and options > may "batch" the journal updates (potentially breaking safety > semantics). Also they do small write when they dequeue the > operations from the journal to the actual metadata records > involved. The ideal config for a journalled filesystem is for put the journal on a separate smaller lower-latency device. e.g. a small RAID1 pair. In a previous work place I had good results with: RAID1 pair of small disks with root, swap, journal Large RAID5/6 array with bulk of filesystem. I also did data journalling as it helps a lot with NFS. > > How bad can this be when the journal is say internal for a > filesystem that is held on wide-stride RAID6 set? I suspect very > very bad, with apocalyptic read-modify-write storms, eating IOPS. > > I suspect that this happens a lot with SSDs too, where the role > of stripe set size is played by the erase block size (often in > the hundreds of KBytes, and even more expensive). > > Where are studies or even just impressions of anedoctes on how > bad this is? > > Are there instrumentation tools in JFS or XFS that may allow me > to watch/inspect what is happening with the journal? For Linux > MD to see what are the rates of stripe r-m-w cases? Not that I am aware of. NeilBrown > -- > 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 -- 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
