Database Reference
In-Depth Information
a client to satisfy the consistency level. Having the two directories on the same
disk may potentially cause the client operation to be blocked. Their I/O patterns
are quite different too. Commit log is basically an append-only write operation,
whereas SSTable is basically random access.
Note
It is important to note that keeping
commitlog
and
data
directories only mat-
ters if you have a spinning disk. For
Solid State Drives
(
SSDs
), you can keep
them on the same disk. However, if you have an SSD and a spinning disk as your
storage devices, it is recommended that you keep commit log on the spinning
disk, and
data
directory on the SSD.
•
RAID 0
: Cassandra performs inbuilt replication by means of a replication factor.
Therefore, it does not possess any sort of hardware redundancy. If one node dies
completely, the data is available on other replica nodes, with no difference
between the two. This is the reason that RAID 0 (
http://en.wikipedia.org/wiki/
RAID#RAID_0
)
is the most preferred RAID level. Another reason is improved
disk performance and extra space.
•
Filesystem
: If one has choices, XFS is the most preferred filesystem for Cas-
sandra deployment. XFS supports 16 TB on a 32-bit architecture, and a whopping
8 EiB (Exabibyte) on 64-bit machines. Owing to storage space limitations, the
ext4
,
ext3
, and
ext2
filesystems (in that order) can be considered to be used
for Cassandra.
•
SCSI and SSD
: With disks, the guideline is the faster, the better. SCSI is faster
than SATA, and SSD is faster than SCSI. SSDs are extremely fast as there are no
moving parts. It is recommended that you use rather low-priced consumer SSD
for Cassandra, as enterprise-grade SSD has no particular benefit over it.
•
No EBS on EC2
: This is specific to
Amazon Web Services
(
AWS
) users.
Elast-
ic Block Store
(
EBS
) from AWS is strongly discouraged for the purpose of stor-
ing Cassandra data—either of data directories or commit log storage. Poor
throughput and issues such as getting unusably slow, instead of cleanly dying, are
major roadblocks of the network attached storage.
•
XFS filesystem
:
http://en.wikipedia.org/wiki/XFS
.
•
AWS EBS
:
http://aws.amazon.com/ebs/
.
Instead of using EBS, use ephemeral
devices attached to the instance (also known as an instance store). Instance stores
are fast and do not suffer any problems as EBS. Instance stores can be configured
as RAID 0 to utilize it even further.
