snapraid/TODO

SnapRAID TODO
=============

This is the list of TODO items for SnapRAID.

- v1.7

* When scanning, ensure that each file is in the expected disk.

* If fix fails for unrecoverable parts, rename such files adding ".unrecoverable".

* Check and fix also empty file.

* When fixing, before overwriting the present file, make a copy of it just in
case that the original file cannot be completely recovered.
We can always open files in read-only mode, if a write is required, we close it,
rename it to with a .bak extension, and rewrite it up to the required size.
The same for symlink if a file with the same name exist or viceversa.

- v1.8

* Check why saving the content files is so slow. fprintf() is the bottleneck.

* Put the content files in the data disks in the default configuration file.

* Easy way to find duplicates using the computed hash.

* Easy way to find files using a specified block.

* Adds a checksum for the content file. When reading if the file is invalid,
skip to another.

* Add an option to exclude "hidden" file. In Windows files with the HIDDEN attributes,
in Unix files starting with '.'.

- v2.x

* Remove the old MD5 support.

* Implement an "undelete" command to recover missing files, without the
integrity check of the existing ones.
It should restore only missing files, without recovering modified ones
or check validity of data of existing ones.

* Rename sync->backup and fix->restore. It seems to me a naming expressing
better the meaning of the commands. But not yet sure.

* Allow to filter the files to check/fix based on the drive where they reside.

* Filtered out files should be opened in read-only mode.

- v3.x

* In the content file, for each block store the last time the hash was checked
with both "check" and also "sync".
It will allow to define a new "check" command checking only aged blocks
and skipping recently checked ones.
This could be problematic for the increased memory occupation, and the need
to update the content file also in check.

* Save the content file in compressed .gz format to save space.

* In the content file save the timestap of the parity files.
If they do not match, stop the processing.
This can be done to avoid to use not syncronized parity and content files,
resulting in wrong data.
But if the sync process is killed we need a way to resyncronize them.


Rejected TODO
=============

This is a list of rejected TODO items.

* Recognizes that a file is moved from one disk to another, and if the parity
data doesn't overlap, do not recompute it.
- It's going to work only in RAID5 mode and only in special cases.

* Uses the O_NOATIME flag if run as 'root'.
- A better solution is to mount the disk with the "noatime" mount option.
Note also that at mount time you can also use the "nodiratime", and this is
the only way to specify it.

* Implements a multithread sync command.
- At now it's questionable if it will result in a performance improvment.
The murmur3 hash, and the RAID5/6 computations are so fast that even a single
thread should be able to do them.
Use the "snapraid -T" comment to see the speed.
Also, all the file operations are already done in background by the OS,
so no improvement is expect from this side.

* The SSE2 RAID6 computation may be slower than the MMX version.
See for example this case on a AMD Zacate APU E-350 1.6GHz:
memset 1812 [MB/s]
MD5 252 [MB/s]
Murmur3 1116 [MB/s]
RAID5 int32x2 1692 [MB/s]
RAID5 mmxx2 1745 [MB/s]
RAID5 sse2x2 2481 [MB/s]
RAID6 int32x2 807 [MB/s]
RAID6 mmxx2 1304 [MB/s]
RAID6 sse2x2 787 [MB/s]
- It happens in some systems. At now not an high priority issue.