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Posted to user@cassandra.apache.org by buddhasystem <po...@bnl.gov> on 2011/03/17 21:09:07 UTC
Does "concurrent_reads" relate to number of drives in RAID0?
Hello, in the instructions, I need to link "concurrent_reads" to number of
drives. Is this related to number of physical drives that I have in my
RAID0, or something else?
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Re: Does "concurrent_reads" relate to number of drives in RAID0?
Posted by buddhasystem <po...@bnl.gov>.
Thanks Peter, I can see it better now.
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Re: Does "concurrent_reads" relate to number of drives in RAID0?
Posted by mcasandra <mo...@gmail.com>.
Also when it comes to RAID controller there are other options like write
policy, read policy, cache io/direct io. Is there any preference on which
policies should be chosen?
In our case:
http://support.dell.com/support/edocs/software/svradmin/1.9/en/stormgmt/cntrls.html
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Re: Does "concurrent_reads" relate to number of drives in RAID0?
Posted by buddhasystem <po...@bnl.gov>.
Where and how do I choose it?
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Re: Does "concurrent_reads" relate to number of drives in RAID0?
Posted by Peter Schuller <pe...@infidyne.com>.
> The reason for this is that you want to be able to saturate your
> storage subsystem, and that means keeping all spindles working at all
> times and efficiently. This is accomplished by ensuring you are able
> to sustain a sufficient queue depth (number of outstanding commands)
> on each device. This in turn, in the case of a RAID0, means
> multiplying the target maximum queue depth with the number of drives.
(But this is all predicated on the operating system actually letting
the I/O requests pass through to the device, which is why I replied
about choosing the deadline or noop scheduler instead of cfq.)
--
/ Peter Schuller
Re: Does "concurrent_reads" relate to number of drives in RAID0?
Posted by Peter Schuller <pe...@infidyne.com>.
> Thanks to all for replying, but frankly I didn't get the answer I wanted.
> Does the "number of disks" apply to number of spindles in RAID0? Or
> something else like a separate disk for commitlog and for data?
The number of actual disks (spindles) in the device on which your
sstables are on (not the commit log).
The reason for this is that you want to be able to saturate your
storage subsystem, and that means keeping all spindles working at all
times and efficiently. This is accomplished by ensuring you are able
to sustain a sufficient queue depth (number of outstanding commands)
on each device. This in turn, in the case of a RAID0, means
multiplying the target maximum queue depth with the number of drives.
--
/ Peter Schuller
Re: Does "concurrent_reads" relate to number of drives in RAID0?
Posted by buddhasystem <po...@bnl.gov>.
Thanks to all for replying, but frankly I didn't get the answer I wanted.
Does the "number of disks" apply to number of spindles in RAID0? Or
something else like a separate disk for commitlog and for data?
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Re: Does "concurrent_reads" relate to number of drives in RAID0?
Posted by Peter Schuller <pe...@infidyne.com>.
> The comment in the example config file next to that setting explains it more
> fully, but something like 16 * number of drives is a reasonable setting for
> readers. Writers should be a multiple of the number of cores.
In addition, if you're running on Linux in a situation where you're
trying to saturate I/O capacity of an underlying device that is an SSD
or a multi-device RAID, I *strongly* suggest switching Linux to the
deadline or noop scheduler. The CFQ scheduler is very poor
out-of-the-box for saturating your I/O subsystem with random reads,
when using SSD:s or RAID controllers with multiple disks.
--
/ Peter Schuller
Re: Does "concurrent_reads" relate to number of drives in RAID0?
Posted by Stu Hood <st...@gmail.com>.
The comment in the example config file next to that setting explains it more
fully, but something like 16 * number of drives is a reasonable setting for
readers. Writers should be a multiple of the number of cores.
On Thu, Mar 17, 2011 at 1:09 PM, buddhasystem <po...@bnl.gov> wrote:
> Hello, in the instructions, I need to link "concurrent_reads" to number of
> drives. Is this related to number of physical drives that I have in my
> RAID0, or something else?
>
> --
> View this message in context:
> http://cassandra-user-incubator-apache-org.3065146.n2.nabble.com/Does-concurrent-reads-relate-to-number-of-drives-in-RAID0-tp6182346p6182346.html
> Sent from the cassandra-user@incubator.apache.org mailing list archive at
> Nabble.com.
>