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I've setup a home RAID for a friend in the past from a heap of old and new 2T and 3T drives. seems very much like what BeyondRAID does. I made a 7 member RAID5 group of the 2T volumes and the first 2TiB of the 3T drives, and a second 5 member group of the upper 1T of the 3T drives, then added those two PVs to an LVM vg and voila, a pretty good maximization of the disk areas, only 3TiB lost on parity.

Now I'm building an array at home. I had 4 disks of 4 TiB (12 TiB usable under RAID5), I then replaced two with the first 4TiB of two 8TiB drives, adding one back as the member of a 3-disk array with the top 4TiB of the two 8T drives and leaving one out of the chasis for emergency rescues. same deal - add the new 8TiB PV to the LVM and I should be good, right?

Well, I have to wonder at this point. I'f I'm losing 8TiB to the parity anyway, does it make more sense to have those two RAID5 groups (with the two 8TB disks members in both).

Other than really long rebuilds if one of the bigger drives fails, this gives me some flexibility I could not have otherwise, like replacing 4TiB drives that fail in the future with 8TB drives. but am I missing anything? It's a very low traffic home media server, made of WD-red because really performance is far from being the bottleneck. Am I missing some basic caveat?

Ira
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    Raid5 is obsolete technology, you shouldn't use it anywhere with today's hard disks. When one member drive fails and is replaced, the rebuild time of parity is simply too long and makes it likely that another drive can fail during rebuild. Also the tricks with several different size drives etc. make this extremely fragile. – Tero Kilkanen Mar 28 '18 at 20:14
  • Thanks @TeroKilkanen! I'm always happy to learn. But what's the alternative you offer? – Ira Mar 29 '18 at 16:57
  • Use hard disks of similar size and RAID-6, that is two parity drives for the complete set. – Tero Kilkanen Mar 30 '18 at 02:35
  • I've considered RAID-6, once I'll save some money to get more drives and another drive chassis, I'm not ruling out converting the RAID from level 5 to level 6, luckily Linux supports that nicely. – Ira Mar 31 '18 at 16:55
  • Don't use RAID-5 for spindles especially for such big drives. Only RAID-10 for spindles. However, you can use RAID-5/6 for SSD's which works fine https://www.starwindsoftware.com/blog/raid-5-was-great-until-high-capacity-hdds-came-into-play-but-ssds-restored-its-former-glory-2 and https://mangolassi.it/topic/16284/raid5-still-the-go-to-setup-for-ssd – Strepsils Apr 13 '18 at 13:36

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Get 4 similar (same?) disks and build RAID10. You won't be happy with parity rebuild times on your 8TB spindles.

BaronSamedi1958
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  • point is I'm trying to make the most of the existing hardware, and losing 50% is above my budget for a 20TiB home media server... – Ira Mar 31 '18 at 16:53
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    In such a case you can do RAID0. With your approach you're going to lose your data either way... – NISMO1968 Mar 31 '18 at 20:09