>>>>> "pw" == Peter Wright <pete@xxx> writes: pw> i think you may be thinking about your software raid pw> configuration data here not metadata... yeah, I am. The meaning of the word ``meta'' is very flexible. The way I've used it _is_ appropriate---in Solaris SVM for example: NAME metadb - create and delete replicas of the metadevice state database (which is where array geometry and state is stored) pw> most decent controllers offer BBU's (Battery Backup Units) pw> which not only allow better I/O rates but also help prevent pw> loss of data during a catastrophic well the BBU's plug the RAID5 write hole, so long as they're not separated from the disks that make up the array, as they would be if for example the hardware RAID controller card failed. They are perhaps sold for speeding up databases and mailservers and (in the old days) NFSv2 servers that do a lot of fsync(), but for the purposes of this ``i wouldn't touch'' thread, again, it's the RAID5 write hole that I care about, not performance. The need for them to achieve the illusion of the correct behavior of a single disk is the reason I think software RAID5 is, AIUI, a bad idea. pw> You think vendors like NetApp/EMC/IBM/etc. use software to pw> implement low level RAID functionality? That's a funny statement, but I know what you mean. In any case I think we agree on this so far as: my criticisms apply to RAID-on-a-card only. The SAN vendors do all have NVRAM that fixes the RAID5 write hole, but the cards often don't. Even cards that say ``we have NVRAM!'' often don't have what the SAN vendors call NVRAM, and that bugs me a lot because they are basing their business on trying to confuse people rather than on building trust, which I think is quite wrong in this space. pw> i'd be willing to bet any problems people have had with pw> hardware RAID may have been due to misconfiguration of the pw> array itself, or a misunderstanding about the fundamentals of pw> configuring RAID. ...well...I think there's a misunderstanding about the fundamental problem of losing your array because you are not able to order the model of hardware RAID controller that matches your metadata, or not being able to safely backup this metadata or move it from one card to another without a lot of hesitant, ominous key-pecks in some clunky BIOS Blue Screen of Setup. (RAID metadata, not filesystem metadata) pw> snap shotting and RAID/mirroring/etc are two completely pw> independent concepts. we disagree. I think the idea that you can use the ability to split a RAID1 to get a very simple snapshot is a relationship between mirroring and snapshots. It's also not an invention of mine nor an odd practice. In fact it's documented in the EXAMPLES section of the gmirror man page.
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