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Re: Speed ups for a disk IO bound machine

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On 12/10/2016 10:05 PM, Piergiorgio Sartor wrote:
> On 2016-12-10 21:44, Peter Chant wrote:
> [...]
>> Well, I've not taken any formal stats but a good part of the load is
>> mariadb at the same time as apache / php and firefox.  I'd assume random.
> 
> This can fit SSD or HDD + cache on SSD.
> 

OK.

> [...]
>> I'm a bit confused here.  My understanding is that RAID10 requires four
>> disks, that they are striped in pairs and the pairs then mirror each
>> other.  So I can't do that if I have two drives.
> 
> Look at the Linux MD RAID-10 documentation.
> You'll see any number, even odd, will do.
> 
> Here some reference:
> 
> https://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10
> 

Thanks.  I've done some reading and there is more to do plus some
experimentation.

I understand that mdadm is used to create the raid arrays, it is not
part of lvm itself?


> In case of RAID-10, with 2 disks, layout "far 2",
> the disks combines RAID-0 and RAID-1.
> 
> So, the RAID will survive a failure of 1 disk,
> but the read performances are of RAID-0.
> 
> In your case, I'll combine the two HDDs in one
> RAID-10 and two SSDs in an other RAID-10.
> Both with layout "far 2".
> 

OK.  Simple to set up with the SSDs as one is blank and the other has
free space.

With RAID-10 far 2 am I correct in assuming that the available capacity
for a two device array of 3TB disks would be 3TB (two copies of data)?


> Part of the RAID-10 with SSD can be used as cache.
>

OK, so having done some reading up but not carried out any testing I
think the following is a possible setup.  Note I am using LUKS so I will
add this extra layer to the mix.  I have not noticed any significant
performance difference with and without it.

SSDs:
Create empty partition on larger system SSD.
Add smaller empty SSD.

Create raid 10 far 2 raid array with mdadm for example (need to check
what metadata means!):
mdadm --create /dev/md-ssd --level=10 --metadata=0.90 --raid-devices=2
--layout=f2 /dev/sda4 /dev/sdb

Use crypt setup to create an encrypted block device on top of this raid
10 far 2 device.

Use LVM to create a volume group on top of the encrypted raid 10 far 2
device.

Create a logical volume on top of the above to use as the cache device.

HDD:
Similar to the SSD case above except that the logical volume will be for
the slow disks doing the bulk of the work.


Create a cached device:
Use lvmcache to create a device using the the two logical volumes
created above (bcache would also work).

Create a file system on top of the cached device.  If using btrfs (what
I do now) use single for data as the raid is occuring a couple of levels
down.

Of course, I could have got completely the wrong idea above and invented
some horrid monster!



It looks like I could do this without the LVM layer between the LUKs and
cache parts.  However, this does give me the flexability to create
logical volumes that are on HDD RAID10, SDD RAID10 soley as well as
cached.  Or different cache options.  I think I could have put another
LVM on top of the cached item I was creating but I think that was over
doing it.


Pete



> [...]
>> Hmm. bcache is simpler as a starting point.
> 
> Well, not really.
> If you already use LVM, than lvmcache is easier.
> Because you can add and remove the cache to the
> LVM volume on the fly.
> With bcache, you'll have to start with it from
> the beginning.
> 
> bye,
> 

Thanks.

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Thread

Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-07 22:41 +0000
  Re: Speed ups for a disk IO bound machine Roger Blake <rogblake@iname.invalid> - 2016-12-08 16:44 +0000
    Re: Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-10 11:24 +0000
      Re: Speed ups for a disk IO bound machine Roger Blake <rogblake@iname.invalid> - 2016-12-11 01:34 +0000
        Re: Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-11 22:07 +0000
  Re: Speed ups for a disk IO bound machine Piergiorgio Sartor <piergiorgio.sartor.this.should.not.be.used@nexgo.REMOVETHIS.de> - 2016-12-10 12:43 +0100
    Re: Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-10 20:44 +0000
      Re: Speed ups for a disk IO bound machine Piergiorgio Sartor <piergiorgio.sartor.this.should.not.be.used@nexgo.REMOVETHIS.de> - 2016-12-10 23:05 +0100
        Re: Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-11 11:54 +0000
          Re: Speed ups for a disk IO bound machine Piergiorgio Sartor <piergiorgio.sartor.this.should.not.be.used@nexgo.REMOVETHIS.de> - 2016-12-11 14:10 +0100
            Re: Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-11 22:46 +0000
              Re: Speed ups for a disk IO bound machine Piergiorgio Sartor <piergiorgio.sartor.this.should.not.be.used@nexgo.REMOVETHIS.de> - 2016-12-12 19:22 +0100
            Re: Speed ups for a disk IO bound machine David Brown <david.brown@hesbynett.no> - 2016-12-13 10:17 +0100
              Re: Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-14 20:03 +0000
                Re: Speed ups for a disk IO bound machine David Brown <david.brown@hesbynett.no> - 2016-12-15 12:55 +0100
                Re: Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-15 23:18 +0000
                Re: Speed ups for a disk IO bound machine David Brown <david.brown@hesbynett.no> - 2016-12-16 09:34 +0100
        Re: Speed ups for a disk IO bound machine David Brown <david.brown@hesbynett.no> - 2016-12-13 09:57 +0100
  Re: Speed ups for a disk IO bound machine David Brown <david.brown@hesbynett.no> - 2016-12-12 09:11 +0100
    Re: Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-12 23:08 +0000
      Re: Speed ups for a disk IO bound machine David Brown <david.brown@hesbynett.no> - 2016-12-13 09:29 +0100
        Re: Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-14 23:16 +0000
          Re: Speed ups for a disk IO bound machine David Brown <david.brown@hesbynett.no> - 2016-12-15 13:15 +0100
            Re: Speed ups for a disk IO bound machine Peter Chant <pete@petezilla.co.uk> - 2016-12-15 22:50 +0000
              Re: Speed ups for a disk IO bound machine David Brown <david.brown@hesbynett.no> - 2016-12-16 09:39 +0100

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