I've uploaded a patch against the latest mdadm (1.5.0): http://parisc-linux.org/~jejb/md_bitmap/mdadm_1_5_0.diff Thanks, Paul Paul Clements wrote: > > Description > =========== > This patch provides the md driver with the ability to track > resync/rebuild progress with a bitmap. It also gives the raid1 driver > the ability to perform asynchronous writes (i.e., writes are > acknowledged before they actually reach the secondary disk). The bitmap > and asynchronous write capabilities are primarily useful when raid1 is > employed in data replication (e.g., with a remote disk served over nbd > as the secondary device). However, the bitmap is also useful for > reducing resync/rebuild times with ordinary (local) raid1, raid5, and > raid6 arrays. > > Background > ========== > This patch is an adjunct to Peter T. Breuer's raid1 bitmap code (fr1 > v2.14, ftp://oboe.it.uc3m.es/pub/Programs/fr1-2.14.tgz). The code was > originally written for 2.4 (I have patches vs. 2.4.19/20 Red Hat and > SuSE kernels, if anyone is interested). The 2.4 version of this patch > has undergone extensive alpha, beta, and stress testing, including a WAN > setup where a 500MB partition was mirrored across the U.S. The 2.6 > version of the patch remains as close to the 2.4 version as possible, > while still allowing it to function properly in the 2.6 kernel. The 2.6 > code has also been tested quite a bit and is fairly stable. > > Features > ======== > > Persistent Bitmap > ----------------- > The bitmap tracks which blocks are out of sync between the primary and > secondary disk in a raid1 array (in raid5, the bitmap would indicate > which stripes need to be rebuilt). The bitmap is stored in memory (for > speed) and on disk (for persistence, so that a full resync is never > needed, even after a failure or reboot). > > There is a kernel daemon that periodically (lazily) clears bits in the > bitmap file (this reduces the number and frequency of disk writes to the > bitmap file). > > The bitmap can also be rescaled -- i.e., change the amount of data that > each bit represents. This allows for increased efficiency at the cost of > reduced bitmap granularity. > > Currently, the bitmap code has been implemented only for raid1, but it > could easily be leveraged by other raid drivers (namely raid5 and raid6) > by adding a few calls to the bitmap routines in the appropriate places. > > Asynchronous Writes > ------------------- > The asynchronous write capability allows the raid1 driver to function > more efficiently in data replication environments (i.e., where the > secondary disk is remote). Asynchronous writes allow us to overcome high > network latency by filling the network pipe. > > Modifications to mdadm > ---------------------- > I have modified Neil's mdadm tool to allow it to configure the > additional bitmap and async parameters. The attached patch is against > the 1.2 mdadm release. Briefly, the new options are: > > Creation: > > mdadm -C /dev/md0 -l 1 -n 2 --persistent --async=512 > --bitmap=/tmp/bitmap_md0_file,4096,5 /dev/xxx /dev/yyy > > This creates a raid1 array with: > > * 2 disks > * a persistent superblock > * asynchronous writes enabled (maximum of 512 outstanding writes) > * bitmap enabled (using the file /tmp/bitmap_md0_file) > * a bitmap chunksize of 4k (bitmap chunksize determines how much data > each bitmap bit represents) > * the bitmap daemon set to wake up every 5 seconds to clear bits in the > bitmap file (if needed) > * /dev/xxx as the primary disk > * /dev/yyy as the backup disk (when asynchronous writes are enabled, the > second disk in the array is labelled as a "backup", indicating that it > is remote, and thus no reads will be issued to the device) > > Assembling: > > mdadm -A /dev/md0 --bitmap=/tmp/bitmap_md0_file /dev/xxx /dev/yyy > > This assembles an existing array and configures it to use a bitmap file. > The bitmap file pathname is not stored in the array superblock, and so > must be specified every time the array is assembled. > > Details: > > mdadm -D /dev/md0 > > This will display information about /dev/md0, including some additional > information about the bitmap and async parameters. > > I've also added some information to the /proc/mdstat file: > > # cat /proc/mdstat > Personalities : [raid1] > md1 : active raid1 loop0[0] loop1[1](B) > 39936 blocks [2/2] [UU] > async: 0/256 outstanding writes > bitmap: 1/1 pages (15 cached) [64KB], 64KB chunk, file: > /tmp/bitmap_md1 > > unused devices: <none> > > More details on the design and implementation can be found in Section 3 > of my 2003 OLS Paper: > http://archive.linuxsymposium.org/ols2003/Proceedings/All-Reprints/Reprint-Clements-OLS2003.pdf > > Patch Location > ============== > > Finally, the patches are available here: > > kernel patch vs. 2.6.2-rc2-bk3 > ------------------------------ > http://parisc-linux.org/~jejb/md_bitmap/md_bitmap_2_30_2_6_2_RC2_BK3_RELEASE.diff > > mdadm patch vs. 1.2.0 > --------------------- > http://parisc-linux.org/~jejb/md_bitmap/mdadm_1_2_0.diff > > So if you're interested, please review, test, ask questions, etc. Any > feedback is welcome. > > Thanks, > Paul - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
