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From Ariel Weisberg <ar...@weisberg.ws>
Subject Re: Best partition type for Cassandra with JBOD
Date Fri, 16 May 2014 19:46:08 GMT
Hi,



Recommending nobarrier (mount option barrier=0) when you don't know if
a non-volatile cache in play is probably not the way to go. A
non-volatile cache will typically ignore write barriers if a given
block device is configured to cache writes anyways.



I am also skeptical you will see a boost in performance. Applications
that want to defer and batch writes won't emit write barriers
frequently and when they do it's because the data has to be there.
Filesystems depend on write barriers although it is surprisingly hard
to get a reordering that is really bad because of the way journals are
managed.



Cassandra uses log structured storage and supports asynchronous
periodic group commit so it doesn't need to emit write barriers
frequently.



Setting read ahead to zero on an SSD is necessary to get the maximum
number of random reads, but will also disable prefetching for
sequential reads. You need a lot less prefetching with an SSD due to
the much faster response time, but it's still many microseconds.



Someone with more Cassandra specific knowledge can probably give better
advice as to when a non-zero read ahead make sense with Cassandra. This
is something may be workload specific as well.



Regards,

Ariel



On Fri, May 16, 2014, at 01:55 PM, Kevin Burton wrote:

That and nobarrier… and probably noop for the scheduler if using SSD
and setting readahead to zero...



On Fri, May 16, 2014 at 10:29 AM, James Campbell
<[1]james@breachintelligence.com> wrote:

Hi all—


What partition type is best/most commonly used for a multi-disk JBOD
setup running Cassandra on CentOS 64bit?


The datastax production server guidelines recommend XFS for data
partitions, saying, “Because Cassandra can use almost half your disk
space for a single file, use XFS when using large disks, particularly
if using a 32-bit kernel. XFS file size limits are 16TB max on a 32-bit
kernel, and essentially unlimited on 64-bit.”


However, the same document also notes that “Maximum recommended
capacity for Cassandra 1.2 and later is 3 to 5TB per node,” which makes
me think >16TB file sizes would be irrelevant (especially when not
using RAID to create a single large volume).  What has been the
experience of this group?


I also noted that the guidelines don’t mention setting noatime and
nodiratime flags in the fstab for data volumes, but I wonder if that’s
a common practice.

James




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