Good day,
I've been a happy user of md RAID5 for over 8 months now. However,a few days ago, one (I wonder which one too) of my (3) WD RE2 750sdecided to start clicking every few seconds during write. No diskshows any abnormalities from others on SMART tests, and raid5sync_action check, checks all right. Seems it's all safe for now, it'sraid5 after all, but that brought to my attention a need, andpossibility, to improve my redundancy, by migrating to RAID6. I wasplanning to grow the array anyways. Therefore, I sit back to google,and encountered a few issues I couldn't really resolve myself.
First of all, what is the current state of RAID5->6 conversion?I've went through a lot of blogs, and it seem that this matter hasbeen considered log ago to be added to mdadm, however I was unable tofind any definitive information about that for now. I've read, wellskimmed, latest mdadm source and I cannot really say I see it there.What's more, I recall reading about future possibility (or lack of it)to change the raid5/6 chunk size, in a way of reshape. I've not foundany updates on that since then. It's mostly out of curiosity, as itseems I'll have to totally recreate my raid anyways.
My second question, is a bit of theoretical one.I've read "The mathematics of RAID-6" by H. Peter Anvin (raid6.pdf).In section 4, he describes a situation of a single disk *corruption*.That makes me wonder, if md implementation, during a sync_actionrepair/check follows that procedure. I remember reading up on raid5,which of course cannot properly detect & correct such things, that allit can do (again, of course) is set a new parity block. That's whyI'm wonder if raid6 repair/check does the same, just setting newparity blocks, or it tries to detect which one is erroneous and takeappropriate actions.
Now, back to the point. What should I do? :) I want to migrate my1.1TiB of data, from 3 hdd RAID5 to, most likely, a 5 hdd RAID6.I guess it's the very last moment, size-of-data-wise, to accommodateall data during migration.
I see the following ways, supposing there's no reshape from raid5to raid6.
1. Copy stuff to 2 new hdds (not raid'ed), create a slightly degradedraid6 out of old 3 hdds (making raid6 looks like it should have had 4),copy stuff over to raid6, and then add/reshape/etc.
2. Create a totally degraded raid6 with 2 new hdds, lacking 2 drives.Copy from current, operational, raid5, and then add/reshape/etc.
3. (risky) Check current raid5, and if it's checks ok, degrade it!Create a 4 hdd raid6 (4 out of 4 operational), copy stuff fromdegraded raid5, add/reshape/etc.
I'll be thankful for any kind of input on that matters.Kind regards,MiKylie// Michał Przyłuski˙ôčş{.nÇ+?ˇ?Ž??+%?Ë˙ąéÝśĽ?w˙ş{.nÇ+?ˇĽ?{ąţś˘wř§ś?ĄÜ¨}Š?˛Ć zÚ&j:+v?¨ţř Żů ŽwĽţ?ŕ2?Ţ?¨čÚ&˘)ߥŤaśÚ˙˙űŕzżäzšŢ?ú+?ů???ݢj˙?wčţf
