[CentOS] Centos 7.0 and mismatched swap file

Chris Murphy lists at colorremedies.com
Mon Feb 16 17:21:38 UTC 2015


On Mon, Feb 16, 2015 at 6:47 AM, Eliezer Croitoru <eliezer at ngtech.co.il> wrote:

> I am unsure I understand what you wrote.
> "XFS will create multiple AG's across all of those
>  devices,"
> Are you comparing md linear/concat to md raid0? and that the upper level XFS
> will run on top them?

Yes to the first question, I'm not understanding the second question.
Allocation groups are created at mkfs time. When the workload IO
involves a lot of concurrency, XFS over linear will beat XFS or ext4
over raid0. Whereas for streaming performance workloads, striped raid
will work better. If redundancy is needed, mdadm permits creation of
1+linear, as compared to 10.
http://xfs.org/docs/xfsdocs-xml-dev/XFS_Filesystem_Structure/tmp/en-US/html/Allocation_Groups.html

You can think of XFS on linear as being something like raid0 at a file
level, rather than at a block level. On a completely empty file system
if you start copying a pile of dozens or more (typically hundreds or
thousands) of files in mail directories, XFS distributes those across
AG's and hence across all drives, in parallel. ext4 would for the most
part focus all writes to the first device until mostly full, then the
2nd device, then the 3rd. And on raid0 you'll get a bunch of disk
contention that isn't really necessary because everyones files are
striped across all drives.

So contrary to popular opinion on XFS being mainly useful for large
files, it's actually quite useful for concurrent read write workflows
of small files on a many disk linear/concat arrangement. This extends
to using raid1 + linear instead of raid10 if some redundancy is
desired.


-- 
Chris Murphy



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