On 8/5/26 06:24, Michael Stone wrote:
On Tue, Aug 04, 2026 at 02:00:05PM -0400, Stefan Monnier wrote:
So overall the circumstances where raid 5/6 makes sense are a small
minority. It's mostly for cases where total capacity is a lot more
important than performance and even there, the benefit is only 33% in
your scenario (8GB vs 6GB). And when total capacity is a lot more
important than performance one would typically go for HDDs (where the
performance impact of raid 5/6 tend to be even more pronounced).
That depends on what you mean by "performance". A large (8 or more
drive) RAID-6 is going to have similar performance to RAID-10 for
random or sequential reads (operations will be balanced across
drives). For sequential writes the RAID-6 will likely be faster.
Random writes will be worse on the RAID-6, but a write cache will hide
much or all of that depending on the details of cache size, write
size, etc. And with larger numbers of drives the difference in
overhead becomes more significant. (50% usable for RAID-10, 75% for 8
drive RAID-6, 83% for 12 drive RAID-6.) And a last really important
feature: RAID-6 can tolerate *two* failed drives in an array without
losing data. With RAID-10 if a drive fails during a rebuild, the array
is done.
This is not entirely true. Suppose we have RAID-10 array which consists
of 4 drives and thus 2 mirror groups. Array will survive the failure of
two drives as long as they will be in the different mirror groups.
But if these two failed drives will be in the same mirror group then the
whole array will be lost.
So RAID-10 gives 2+ drive fault tolerance as long as the failed drives
are each in the different mirror groups. The smarter guys than me could
calculate the probability.
And rebuilds can take a *long* time (like, days).
You probably thinking about RAID-0+1 array configuration, which is not
RAID-10.
Suppose we have 8x4TB drives. In case of a single drive failure the
RAID-0+1 array would have to rebuild the whole 16TB worth of data (one
mirror group of 4 drives) which indeed could take a long time on a busy
server,
but with the RAID-10 array it will rebuild one mirror group worth of
data (4TB) which is the size of single drive.
RAID-5 and RAID-6 rebuild of 8 drive array would take a very long time,
because not only it will have to recover (not just copy) the data from
error correction data spread across all drives, it also will have to
rebuild and recalculate the error correction data for the failed drive
for the entire array.
You can get double disk failure tolerance on RAID-10, but then you're
down to 33% usable space.
This statement looks false to me, can you elaborate?
AFAIK the RAID-10 array configuration, build from even number of devices
(4 drives or more), will always give at least 2 drive fault tolerance,
except in the situation I've described above, and the usable space for
the 4 drives RAID-10 array always will be 50%.
--
With kindest regards, Alexander.
Debian - The universal operating system
https://www.debian.org
