Re: difference between striping using mdadm and LVM

Aragorn <aragorn@xxxxxxxxxxxxxxxxxxx> wrote in news:hj1gta$2hp$5
@news.eternal-september.org:

Thanks for the great explaination!

Writing to a RAID 5 is slower than writing to a single disk because with
each write, the parity block must be updated, which means calculation
of the parity data and writing that parity data to the pertaining disk.

This is where I get confused. Is writing to a RAID5 slower than a single
disk irrespective of how many disks I throw at the RAID5? I currently have
a 7-disk RAID5. Will writing to this be slower than a single disk? Isn't
the parity calculation a fairly fast process especially if one has a
hardware based card? And then if the write gets split into 6 parts shouldnt
that speed up the process since each disk is writing only 1/6th of the
chunk?


In this case, you don't have any redundancy. Writing to the stripeset
is faster than writing to a single disk, and the same applies for
reading. It's not a 2:1 performance boost due to the overhead for
splitting the data for writes and re-assembling it upon reads, but
there is a significant performance improvement, and especially so if
you use more than two disks.

Why doesn;t a similar boost come out of a RAID5 with a large number of
disks? Merely because of the parity calculation overhead?



There are however a few considerations you should take into account with
both of these approaches, i.e. that you should not put the filesystem
which holds the kernels and /initrd/ - and preferably not the root
filesystem either[1] - on a stripe, because the bootloader recognizes

Luckily that is not needed. I have a seperate drive to boot from. The RAID
is intended only for user /home dirs.

--
Rahul
.

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