A chunk is the unit of data that the MediaAgent software uses to store data on
media. For Sequential access media, chunk is defined as data between two file markers.
The default chunk size for File System like data is 4 GB and for data associated
with databases is 16 GB. For Random access media each chunk is a file on the disk.
The default chunk size for this type of media is 2 GB.
This parameter will have an impact only when backing up to tape. A higher chunk
size will give you better throughput. Recommended values are: 8 GB, 16 GB or 32
GB. However the disadvantage is that granular restores (e.g., single file restore)
will be slower. On the other hand large restores, like a full machine rebuild will
be a bit faster.
A lower value is recommended for frequent checks of slower data protection
operations, especially when data is moving across a WAN link.
Chunk size is configurable using the Chunk Size tab of the Media
Management Configuration dialog box, on a per application basis. The
Media Management Configuration dialog box is accessible from the
CommCell Console Control
panel.(See
Set the Chunk Size for
Each Agentfor step-by-step instructions.)
The chunk size established
in this dialog box affects all data write operations in the CommCell. However
you can establish the
DMMBCHUNKSIZE
registry key to control the chunk size of data write operations going to
the MediaAgent in which the registry key is created. Note that if this registry
key is created it will override the values established in this dialog box
for that MediaAgent.
Also note that the values established for the NAS NDMP
iDataAgents in this dialog box
only apply to libraries attached to the MediaAgent using NDMP Remote Server.
For libraries attached to a File Server, the system, by default, uses 4GB
as the chunk size for file servers that support restartability (e.g., NetApp
filer). For such file servers you can establish the
nRESTARTWINDOWSIZE
registry key to control the chunk size. For other file servers, the chunk
size is based on the size of the tape used to backup the data and this is
not configurable.
(The values in the Chunk Size tab of the Media Management Configuration
dialog box applies only to tape media; for and optical media
the system by default uses 2 GB as the chunk size.)
Chunk Size can also be established from the Data Path Properties
dialog box available from the Data Paths tab of the Copy Properties
dialog box, for the specific data path. (See
Set the Chunk
Size and Block Size for a Data Path for step-by-step instructions.)
Chunk size established at the data path level overrides the chunk sizes
established for the various Agents, as discussed in the preceding paragraph.
MediaAgents can write to media using different block sizes, if the Operating
System associated with the MediaAgent in which the library is configured
supports a higher block size. The system can write block sizes up to 256 KB and
can automatically read block sizes up to 512 KB. If the block sizes are larger
than 512 KB, read operations from the media will fail. Also note that such media
will be over written and re-used if the When Content
Verification Failed option is enabled in the
Library Properties (Media) dialog box
Block sizes can be modified from the
Data Path Properties dialog box available from the
Data Paths tab of the
Copy Properties dialog box, for the
specific data path. (See
Set the Chunk Size and
Block Size for a Data Pathfor
step-by-step instructions.)
Higher block sizes on tape media are faster, but consider the following
before increasing the block size:
Note that the new block size must be supported by the HBA driver
as well as by the tape drive. The most commonly used values are 128
KB or 256 KB.
MediaAgents with direct attached SCSI devices on Windows 2000 do
not support block sizes higher than 64 KB.
NetApp NAS file servers for ONTAP 6.5.3 or later allow block sizes
up to 256 KB, but NetApp servers for ONTAP 6.5.2 or earlier allow maximum
block sizes of 64 KB.
EMC Celerra NAS file servers support block sizes up to 128 KB.
If you do specify a block size larger than the allowed limit, the
NAS backup process automatically changes your entry to the maximum allowable
block size.
For mount paths, block size is the buffer size used for each
write operation. (The File system allocation unit size is decided at the
time of formatting the volume.)
Cautions:
All MediaAgents that share a storage policy must support the higher
block size.
If the MediaAgent that performs the restore is different than the
original MediaAgent, then the destination MediaAgent must have HBA cards
and drives to be able to read the higher block size.
The system, by default updates the Job Manager, every 5 minutes or whenever a
chunk is closed, whichever is sooner. You can marginally improve the performance
by increasing the job update interval when you increase the chunk size. However
increasing the update interval will result in infrequent updates to the job progress
in the Job Controller. These changes will not impact NAS NDMP
iDataAgent backups.
Application read size is the size of the application data read from the
clients for data transfer during backup operations. This parameter is defined
for each subclient in the
Storage Device
(Data Transfer Option) tab. This value is set at 64 KB by default. Values
for Application Read size must be in the power of 2; minimum value is 64 KB,
maximum value is 4096 (4 MB).
Increasing the application read size will increase the rate of data transfer. Note that each application internally allocates a buffer size that is
suitable for handling the application data. When the size of the application
data read during backup operations matches the source application's internal
buffer allocation then the overhead is minimized. So to achieve optimal rate of
data transfer during backups, configure this value based on the source application's buffer
allocation. See
Set the Application Read Size
for step-by-step instructions.
This configuration is supported for the following agents:
When backing up to devices, you can enable Use Unbuffered I/O
option in the Mount Path Properties dialog box to bypass the Windows file
system buffering. Varying amounts of speedup will be observed depending upon the
architecture of the disk. (See
Use Unbuffered
I/O for additional information.)
You can establish the
dwMaxAsyncIoRequests
registry key to control the number of read ahead buffers in the unbuffered I/O mode.
The read streams for deduplication jobs are not
unbuffered. Therefore, this registry key does not have an effect on
deduplicated data and in completion of Auxiliary copy jobs of
deduplication data sets.
Multiplexing does not improve performance of an individual backup operation.
However multiple backups run in parallel to a single tape drive, results in better
utilization of the tape drives, especially when the backups are from slower clients.
This helps in better overall throughput and reduction in the backup window.
For LAN backups, make sure that the network between the clients and MediaAgent
is capable of supporting multiple simultaneous backups.
Do not over multiplex. That would be counter-productive and slow down the backups
as well as restores. Multiplexing factor must be set equal to the ratio of tape
drive throughput and client source speed. For example, if the tape drive has rated
speed of 40 Mb/sec and clients are able to supply the data at about 12 Mb/sec, then
a multiplexing factor of 3 is advisable. Typical multiplexing factor is between
2 and 5.
Also note that restores from multiplexed data are not slower.
