Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

write. However, in the event of a server crash, when the server has not flushed
the new data to the newly allocated blocks, the data seen on the overwritten region
would be similar to what you would find in the case of an allocating write where
the server has crashed before the data is flushed. This is not the default behavior
and with the default behavior the data that you find in the overwritten region will
be either the new data or the old data.
Reading from FileSnaps
For regular read requests, Veritas File System (VxFS) only caches a single copy
of a data page in the page cache for a given shared data block, even though the
shared data block could be accessed from any of the FileSnaps or the source file.
Once the shared data page is cached, any subsequent requests via any of the
FileSnaps or the source file is serviced from the page cache. This eliminates
duplicate read requests to the disk, which results in lower I/O load on the array.
This also reduces the page cache duplication, which results in efficient usage of
system page cache with very little cache churning when thousands of FileSnaps
are accessed.
Block map fragmentation and FileSnaps
The block map of the source file is shared by the snapshot file. When data is
overwritten on a previously shared region, the block map of the file to which the
write happens gets changed. In cases where the shared data extent of a source
file is larger than the size of the overwrite request to the same region, the block
map of the file that is written to becomes more fragmented.
Backup and FileSnaps
A full backup of a VxFS file system that has shared blocks may require as much
space in the target as the number of total logical references to the physical blocks
in the source file system.
For example, if you have a 20 GB file from which one thousand FileSnaps were
created, the total number of logical block references is approximately 20 TB. While
the VxFS file system only requires a little over 20 GB of physical blocks to store
the file and the file's one thousand snapshots, the file system requires over 20 TB
of space on the backup target to back up the file system, assuming the backup
target does not have deduplication support.
Administering FileSnaps
Reading from FileSnaps
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