Hi. Well for me personally these are follow up questions to my scenario presented here: http://thread.gmane.org/gmane.linux.raid/43405 But I think these questions would be generally interesting an I'd like to add them to the Debian FAQ for mdadm (and haven't found real good answers in the archives/google). 1) I plan to use dmcrypt and LUKS and had the following stacking in mind: physical devices -> MD -> dmcrypt -> LVM (with multiple LVs) -> filesystems Basically I use LVM for partitioning here ;-) Are there any issues with that order? E.g. I've heard rumours that dmcrypt on top of MD performs much worse than vice versa... But when looking at potential disaster recovery... I think not having MD directly on top of the HDDs (especially having it above dmcrypt) seems stupid. 2) Chunks / Chunk size a) How does MD work in that matter... is it that it _always_ reads and/or writes FULL chunks? Guess it must at least do so on _write_ for the RAID levels with parity (5/6)... but what about read? And what about read/write with the non-parity RAID levels (1, 0, 10, linear)... is the chunk size of any real influence here (in terms of reading/writing)? b) What's the currently suggested chunk size when having a undetermined mix of file sizes? Well it's obviously >= filesystem block size... dm-crypt blocksize is always 512B so far so this won't matter... but do the LVM physical extents somehow play in (I guess not,... and LVM PEs are _NOT_ always FULLY read and/or written - why should they? .. right?) From our countless big (hardware) RAID systems at the faculty (we run a Tier-2 for the LHC Computing Grid)... experience seems that 256K is best for an undetermined mixture of small/medium/large files... and the biggest possible chunk size for mostly large files. But does the 256K apply to MD RAIDs as well? 3) Any extra benefit from the parity? What I mean is... does that parity give me kinda "integrity check"... I.e. when a drive fails completely (burns down or whatever)... then it's clear... the parity is used on rebuild to get the lost chunks back. But when I only have block errors... and do scrubbing... a) will it tell me that/which blocks are damaged... it will it be possible to recover the right value by the parity? Assuming of course that block error/damage doesn't mean the drive really tells me an error code for "BLOCK BROKEN"... but just gives me bogus data? Thanks again, Chris.
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