Re: [gentoo-amd64] Re: Is my RAID performance bad possibly due to starting sector value?

[Matthew Marlowe <matt@professionalsysadmin.com>]

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I would recommend that anyone concerned about mdadm software raid
performance on gentoo test via tools like bonnie++ before putting any data
on the drives and separate from data into different sets/volumes.

I did testing two years ago watching read, write burst and sustained rates,
file ops per second, etc.... Ended up getting 7 2tb enterprise data drives
Disk 1 is os, no raid
Disk 2-5 are data, raid 10
Disk 6-7 are backups and to test/scratch space, raid 0
On Jun 22, 2013 4:04 PM, "Mark Knecht" <markknecht@gmail.com> wrote:

> On Fri, Jun 21, 2013 at 3:28 AM, Rich Freeman <rich0@gentoo.org> wrote:
> > On Fri, Jun 21, 2013 at 3:31 AM, Duncan <1i5t5.duncan@cox.net> wrote:
> >> So with 4k block sizes on a 5-device raid6, you'd have 20k stripes, 12k
> >> in data across three devices, and 8k of parity across the other two
> >> devices.
> >
> > With mdadm on a 5-device raid6 with 512K chunks you have 1.5M in a
> > stripe, not 20k.  If you modify one block it needs to read all 1.5M,
> > or it needs to read at least the old chunk on the single drive to be
> > modified and both old parity chunks (which on such a small array is 3
> > disks either way).
> >
>
> Hi Rich,
>    I've been rereading everyone's posts as well as trying to collect
> my own thoughts. One question I have at this point, being that you and
> I seem to be the two non-RAID1 users (but not necessarily devotees) at
> this time, is what chunk size, stride & stripe width with you are
> using? Are you currently using 512K chunks on your RAID5? If so that's
> potentially quite different than my 16K chunk RAID6. The more I read
> through this thread and other things on the web the more I am
> concerned that 16K chunks has possibly forced far more IO operations
> that really makes sense for performance. Unfortunately there's no easy
> way to me to really test this right now as the RAID6 uses the whole
> drive. However for every 512K I want to get off the drive you might
> need 1 chuck whereas I'm going to need what, 32 chunks? That's got to
> be a lot more IO operations on my machine isn't it?
>
>    For clarity, I'm a 16K chunk, stride of 4K, stripe of 12K:
>
> c2RAID6 ~ # tune2fs -l /dev/md3 | grep RAID
> Filesystem volume name:   RAID6root
> RAID stride:              4
> RAID stripe width:        12
> c2RAID6 ~ #
>
> c2RAID6 ~ # cat /proc/mdstat
> Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
> md3 : active raid6 sdb3[9] sdf3[5] sde3[6] sdd3[7] sdc3[8]
>       1452264480 blocks super 1.2 level 6, 16k chunk, algorithm 2 [5/5]
> [UUUUU]
>
> unused devices: <none>
> c2RAID6 ~ #
>
>    As I understand one of your earlier responses I think you are using
> 4K sector drives, which again has that extra level of complexity in
> terms of creating the partitions initially, but after that should be
> fairly straight forward to use. (I think) That said there are
> trade-offs between RAID5 & RAID6 but have you measured speeds using
> anything like the dd method I posted yesterday, or any other way that
> we could compare?
>
>    As I think Duncan asked about storage usage requirements in another
> part of this thread I'll just document it here. The machine serves
> main 3 purposes for me:
>
> 1) It's my day in, day out desktop. I run almostly totally Gentoo
> 64-bit stable unless I need to keyword a package to get what I need.
> Over time I tend to let my keyworded packages go stable if they are
> working for me. The overall storage requirements for this, including
> my home directory, typically don't run over 50GB.
>
> 2) The machine runs 3 Windows VMs every day - 2 Win 7 & 1 Win XP.
> Total storage for the basic VMs is about 150GB. XP is just for things
> like NetFlix. These 3 VMs typically have allocated 9 cores allocated
> to them (6+2+1) leaving 3 for Gentoo to run the hardware, etc. The 6
> core VM is often using 80-100% of its CPUs sustained for times. (hours
> to days.) It's doing a lot of stock market math...
>
> 3) More recently, and really the reason to consolidate into a single
> RAID of any type, I have about 900GB of mp4s which has been on an
> external USB drive, and backed up to a second USB drive. However this
> is mostly storage. We watch most of this video on the TV using the
> second copy drive hooked directly to the TV or copied onto Kindles.
> I've been having to keep multiple backups of this outside the machine
> (poor man's RAID1 - two separate USB drives hooked up one at a time!)
> ;-) I'd rather just keep it safe on the RAID 6, That said, I've not
> yet put it on the RAID6 as I have these performance issues I'd like to
> solve first. (If possible. Duncan is making me worry that they cannot
> be solved...)
>
>    Lastly, even if I completely buy into Duncan's well formed reasons
> about why RAID1 might be faster, using 500GB drives I see no single
> RAID solution for me other than RAID5/6. The real RAID1/RAID6
> comparison from a storage standpoint would be a (conceptual) 3-drive
> RAID6 vs 3 drive RAID1. Both create 500GB of storage and can
> (conceptually) lose 2 drives and still recover data. However adding
> another drive to the RAID1 gains you more speed but no storage (buying
> into Duncan's points) vs adding storage to the RAID6 and probably
> reducing speed. As I need storage what other choices do I have?
>
>    Answering myself, take the 5 drives, create two RAIDS - a 500GB
> 2-drive RAID1 for the system + VMs, and then a 3-drive RAID5 for video
> data maybe? I don't know...
>
>    Or buy more hardware and do a 2 drive SSD RAID1 for the system, or
> a hardware RAID controller, etc. The options explode if I start buying
> more hardware.
>
>    Also, THANKS TO EVERYONE for the continued conversation.
>
> Cheers,
> Mark
>
>

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