Disk image band files

[Hartie]

Disk Images uses 50mb band-files. If i write 1mb file to dmg, does it actually write entire 50mb file? I ask this because SSD-disks can write limited times and i consider moving some of my data out of dmg to limit writes.

[agentx]

I think Apple sparse disk image files are 8MB "bands" so maybe use that.

[DeltaMac]

50mb (50MB)? Do you have a source for that particular value? I don't think I've ever seen any info about 'banding', except for sparse image files.

[chabig]

Quote: Originally Posted by Hartie I ask this because SSD-disks can write limited...

You will probably never wear out your SSD:

http://www.anandtech.com/show/2829/6

[Hartie]

Quote: Originally Posted by DeltaMac 50mb (50MB)? Do you have a source for that particular value? I don't think I've ever seen any info about 'banding', except for sparse image files.

I used some utility in my Mac and it showed that my recently created dmg had 50mb bands. It might be sparse image file.

So if i write 1mb file to dmg, does it actually write entire 50mb file?

Thanks

[chabig]

Quote: Originally Posted by DeltaMac 50mb (50MB)? Do you have a source for that particular value? I don't think I've ever seen any info about 'banding', except for sparse image files.

Sparce BUNDLES are banded. It's easy to see how large the bands are...just open the bundle in Finder and check. I have a Sparse Bundle that's 3.87GB and it's divided into 463 bands of 8MB each. I also have a small Sparse Bundle of 102MB and it's also divided into bands of 8MB (13 of them).

[chabig]

It's 8MB bands by default. I found the reference in the hdiutil man page:

Quote: -type UDIF|SPARSE|SPARSEBUNDLE -type is particular to create and is used to specify the format of empty read/write images. It is independent of -format which is used to specify the final read-only image format when populating an image with pre-existing content. UDIF is the default type. If specified, a UDRW of the specified size will be cre- ated. SPARSE creates a UDSP: a read/write single-file image which expands as is is filled with data. SPARSEBUNDLE creates a UDSB: a read/write image backed by a directory bundle. By default, UDSP images grow one megabyte at a time. Introduced in 10.5, UDSB images use 8 MB band files which grow as they are written to. -imagekey sparse-band-size=size can be used to specify the number of 512-byte sectors that will be added each time the image grows. Valid values for SPARSEBUNDLE range from 2048 to 16777216 sectors (1 MB to 8 GB). The maximum size of a SPARSE image is 128 petabytes; the maximum for SPARSEBUNDLE is just under 8 exabytes (2^63 - 512 bytes minus 1 byte). The amount of data that can be stored in either type of sparse image is additionally bounded by the filesystem in the image and by any partition map. compact can reclaim unused bands in sparse images backing HFS+ filesystems. resize will only change the vir- tual size of a sparse image. See also USING PERSISTENT SPARSE IMAGES below.

[acme.mail.order]

Quote: Originally Posted by Hartie Disk Images uses 50mb band-files. If i write 1mb file to dmg, does it actually write entire 50mb file?

It will write as much as it needs - if your addition crosses a band boundary by a single byte then another band gets created. The extra space won't necessarily get written to, but it will be allocated.

[fracai]

To add a bit to this, I recently ran a batch of tests that seemed to indicate that SparseBundles are pretty intelligent about what data gets written. While I haven't written anything up, the basics are that I created bundles with varying sizes of bands and then wrote to those over a network. They pretty much all had the same performance, so I can see only two advantages to using different band sizes.

1) Smaller bands will provide more opportunity for reclaiming old space with "diskutil compact"
2) Larger bands will minimize the number of bands required to represent a volume. This could be important if you try to fill a 1TB bundle using 1MB bands and the system has trouble reading the bands directory.