On 2017-11-16 07:33, Zdenek Kabelac wrote:
Dne 16.11.2017 v 11:04 Qu Wenruo napsal(a):
On 2017年11月16日 17:43, Zdenek Kabelac wrote:
Dne 16.11.2017 v 09:08 Qu Wenruo napsal(a):
[What we have]
The nearest infrastructure I found in kernel is
bio_integrity_payload.
Hi
We already have dm-integrity target upstream.
What's missing in this target ?
If I didn't miss anything, the dm-integrity is designed to calculate and
restore csum into its space to verify the integrity.
The csum happens when bio reaches dm-integrity.
However what I want is, fs generate bio with attached verification hook,
and pass to lower layers to verify it.
For example, if we use the following device mapper layout:
FS (can be any fs with metadata csum)
|
dm-integrity
|
dm-raid1
/ \
disk1 disk2
If some data in disk1 get corrupted (the disk itself is still good), and
when dm-raid1 tries to read the corrupted data, it may return the
corrupted one, and then caught by dm-integrity, finally return -EIO to
FS.
But the truth is, we could at least try to read out data in disk2 if we
know the csum for it.
And use the checksum to verify if it's the correct data.
So my idea will be:
FS (with metadata csum, or even data csum support)
| READ bio for metadata
| -With metadata verification hook
dm-raid1
/ \
disk1 disk2
dm-raid1 handles the bio, reading out data from disk1.
But the result can't pass verification hook.
Then retry with disk2.
If result from disk2 passes verification hook. That's good, returning
the result from disk2 to upper layer (fs).
And we can even submit WRITE bio to try to write the good result back to
disk1.
If result from disk2 doesn't pass verification hook, then we return -EIO
to upper layer.
That's what btrfs has already done for DUP/RAID1/10 (although RAID5/6
will also try to rebuild data, but it still has some problem).
I just want to make device-mapper raid able to handle such case too.
Especially when most fs supports checksum for their metadata.
Hi
IMHO you are looking for too complicated solution.
If your checksum is calculated and checked at FS level there is no added
value when you spread this logic to other layers.
dm-integrity adds basic 'check-summing' to any filesystem without the
need to modify fs itself - the paid price is - if there is bug between
passing data from 'fs' to dm-integrity' it cannot be captured.
But that is true of pretty much any layering, not just dm-integrity.
There's just a slightly larger window for corruption with dm-integrity.
Advantage of having separated 'fs' and 'block' layer is in its
separation and simplicity at each level.
If you want integrated solution - you are simply looking for btrfs where
multiple layers are integrated together.
You are also possibly missing feature of dm-interity - it's not just
giving you 'checksum' - it also makes you sure - device has proper
content - you can't just 'replace block' even with proper checksum for a
block somewhere in the middle of you device... and when joined with
crypto - it makes it way more secure...
And to expand a bit further, the correct way to integrate dm-integrity
into the stack when RAID is involved is to put it _below_ the RAID
layer, so each underlying device is it's own dm-integrity target.
Assuming I understand the way dm-raid and md handle -EIO, that should
get you a similar level of protection to BTRFS (worse in some ways,
better in others).
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