Oracle Video server, Manual

Page: 46 / 78

Oracle Video Server (OVS)
2-10 Introducing Oracle Video Server
The RAID stripe is the smallest unit of allocation in the MDS.
If a disk fails, the MDS can dynamically reconstruct its data from the other disks in
the RAID set, using the parity information. The reconstruction occurs as the data is
requested and is transparent to the user. You can also rebuild data from a failed
disk onto a new disk with the rebuild utility.
The MDS can tolerate one failed disk in each RAID set and continue to deliver
video without glitches, or disturbances in the video. If there are multiple
concurrent failures in a RAID set, the missing data cannot be reconstructed and the
volume can no longer play video.
Space Allocation When writing a file to a volume, the MDS writes each stripe
sequentially across disks in each RAID set and writes each RAID stripe
sequentially across RAID sets in the volume.
A volume’s table of contents is stored on the first RAID stripes of the first few
RAID sets, depending on how the volume is defined. The MDS begins storing files
in an empty volume with the first RAID stripe on the RAID set after the table of
contents. The MDS writes sequentially across disks in the RAID set, writing one
stripe on each disk. After filling a RAID stripe, the MDS writes sequentially across
the first RAID stripe in the next RAID set, and so on. When the MDS reaches the
last disk in the last RAID set, it returns to the first RAID set and proceeds to write
to the second RAID stripe. The last RAID stripe allocated for the file may not be
completely filled by the file. This space remains empty. To write another file to the
volume, the MDS begins with the first disk in the next RAID set.
The number of stripes taken up by a file depends on the length of the file as
illustrated in Figure 2–6. Each RAID stripe is labeled by the file containing it (A, B,
or C) and the number of the RAID stripe within the file. Note that the figure does
not distinguish data from parity information. All RAID stripes, including the one
containing directory structure information, include an associated parity stripe.
The space allocated for a file may exceed the actual file size by:
(stripesize x (RAIDsize–1))-1
for a 1 byte file.
If a file is deleted from a volume, the MDS can reclaim that file’s space by storing
one or more new files that fit there. The MDS cannot distribute a file over
noncontiguous RAID stripes. The defragmenter utility can make the
noncontiguous empty RAID stripes contiguous.