Biomedical Engineering Reference
In-Depth Information
10.7.2 Disk arrays
All disk arrays operate at RAID 6 level. In technical terms, this means
this provides 'block level striping with double distributed parity'; in
practical terms, an array can lose up to two disks without any data loss.
In the event of a disk failure (which has happened more than once), the
faulty disk can be replaced with a new disk. The disk array automatically
re-builds and the hardware is repaired.
Lesson: Buy spare disks when you buy the hardware to ensure
compatibility or replacements.
A further enhancement is the use of hot spares. This is a disk physically
installed in the array but in an inactive state. If a disk fails, the system
automatically replaces the failed drive with a hot spare and re-builds the
array. The advantage of this is that it reduces the recovery time. However,
it can still take many hours for a disk to re-build; in our disk arrays 24
hours would normally be allowed for a disk to complete re-building. If
there is a fault with the unit other than with a disk, then downtime will
be longer. One of the advantages gained from having duplicate NAS
boxes is that, in the event of a such a fault, we can switch over from the
primary NAS to the secondary unit in order to maintain a functional
system. With the new duplicate NAS boxes should there be a fault with
the unit as opposed to a disk, then we can switch to the backup NAS box
while the primary unit can be repaired.
10.7.3 Data
Besides ensuring that the hardware is stable, the data on the disks can
also be secured using backups. It is worth considering what data
absolutely must be saved and what could be lost. For example, prioritising
the TIFF image fi les generated by the scanners from the processed
microarray slides. All the data used in the analysis of the microarray are
extracted from the image fi le and so can easily be regenerated as long as
the original TIFF image fi le is available.
An additional measure we take is in backing up relational databases.
The MySQL administration command mysqldump provides a useful
mechanism to create an archive of all the information in the database.
The resultant fi le contains everything required to recreate the database
and restore any lost data. Similarly, all software development is managed
with Subversion [8], a software versioning and revision control system.
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