>>>>> "pw" == Peter Wright <pete@xxx> writes: pw> well that's why any hardware raid controller worth purchasing pw> supports BBU's and a write-back cache. IIRC most Dell PERC are sold without pw> otherwise you would need to use synchronous writes when pw> mounting the filesystem, which for some people may be pw> acceptable. I disagree. The NVRAM may allow you to turn on -o sync without losing as much performance w.r.t. softdep as you would with a real disk. but if you use -o softdep like a normal mount, (if it weren't for the problems of NVRAMless RAID5) you would have the same level of integrity protection with or without the BBU---that level which is defined by softdep. And on a non-NVRAM system, mounting the filesystem '-o sync' will *not* help with the RAID5 write hole. only NVRAM will help that. pw> this is an important distinction: if a RAID implementation is pw> tied to the OS, then any interruption to the OS increases risk pw> a data corruption. by offloading this to a ASIC with a BBU pw> one mitigates this risk by allowing data in caches to be pw> sync'd to disk regardless of the sate of the OS. but softdep does provide integrity guarantees on power loss. and using RAID3 instead of RAID5 closes the RAID5 write hole by always doing full stripe writes (ZFS does the same thing). pw> i guess i'm just not sure what you mean by "RAID-on-a-card". I mean so-called ``hardware RAID'' that's implemented by a tiny computer on a PCI card or a motherboard. This is what most people seem to run to after they use software RAID5 and lose a bunch of data, or get tired of waiting for mirror rebuilds with software RAID1. and it is IMHO junk. pw> i still don't see your point, If you smoke your RAID card, and it's an old RAID card, and you can't get another one, you can lose the contents whole array. Sometimes, even if you can get another one, some quirk of the multiple BIOS/DOS/Windows configuraturs they give you makes it impossible to get the old array working with your new card. I've heard multiple stories like this from friends, and the Interweb is full of them as well with headings like ``Dell PERC OH GOD NO''. Software RAID doesn't have this problem. (nor does SAN under a service contract.) It has other problems, but not this one.
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