Re: [Qemu-devel] [RFC V8 01/24] qcow2: Add journal specification.

[Benoît Canet]

> > +QCOW2 can use one or more instance of a metadata journal.
> 
> s/instance/instances/
> 
> Is there a reason to use multiple journals rather than a single journal
> for all entry types?  The single journal area avoids seeks.

Here are the main reason for this:

For the deduplication some patterns like cycles of insertion/deletion could
leave the hash table almost empty while filling the journal.

If the journal is full and the hash table is empty a packing operation is
started.

Basically a new journal is created and only the entry presents in the hash table
are reinserted.

This is why I want to keep the deduplication journal appart from regular qcow2
journal: to avoid interferences between a pack operation and regular qcow2
journal entries.

The other thing is that freezing the log store would need a replay of regular
qcow2 entries as it trigger a reset of the journal.

Also since deduplication will not work on spinning disk I discarded the seek
time factor.

Maybe commiting the dedupe journal by erase block sized chunk would be a good
idea to reduce random writes to the SSD.

The additional reason for having multiple journals is that the SILT paper
propose a mode where prefix of the hash is used to dispatch insertions in
multiples store and it easier to do with multiple journals.

> 
> > +
> > +A journal is a sequential log of journal entries appended on a previously
> > +allocated and reseted area.
> 
> I think you say "previously reset area" instead of "reseted".  Another
> option is "initialized area".
> 
> > +A journal is designed like a linked list with each entry pointing to the 
> > next
> > +so it's easy to iterate over entries.
> > +
> > +A journal uses the following constants to denote the type of each entry
> > +
> > +TYPE_NONE = 0xFF      default value of any bytes in a reseted journal
> > +TYPE_END  = 1         the entry ends a journal cluster and point to the 
> > next
> > +                      cluster
> > +TYPE_HASH = 2         the entry contains a deduplication hash
> > +
> > +QCOW2 journal entry:
> > +
> > +    Byte 0         :    Size of the entry: size = 2 + n with size <= 254
> 
> This is not clear.  I'm wondering if the +2 is included in the byte
> value or not.  I'm also wondering what a byte value of zero means and
> what a byte value of 255 means.

I am counting the journal entry header in the size. So yes the +2 is in the byte
value.
A byte value of zero, 1 or 255  is an error.

Maybe this design is bogus and I should only count the payload size in the size
field. It would make less tricky cases.

> 
> Please include an example to illustrate how this field works.
> 
> > +
> > +         1         :    Type of the entry
> > +
> > +         2 - size  :    The optional n bytes structure carried by entry
> > +
> > +A journal is divided into clusters and no journal entry can be spilled on 
> > two
> > +clusters. This avoid having to read more than one cluster to get a single 
> > entry.
> > +
> > +For this purpose an entry with the end type is added at the end of a 
> > journal
> > +cluster before starting to write in the next cluster.
> > +The size of such an entry is set so the entry points to the next cluster.
> > +
> > +As any journal cluster must be ended with an end entry the size of regular
> > +journal entries is limited to 254 bytes in order to always left room for 
> > an end
> > +entry which mimimal size is two bytes.
> > +
> > +The only cases where size > 254 are none entries where size = 255.
> > +
> > +The replay of a journal stop when the first end none entry is reached.
> 
> s/stop/stops/
> 
> > +The journal cluster size is 4096 bytes.
> 
> Questions about this layout:
> 
> 1. Journal entries have no integrity mechanism, which is especially
>    important if they span physical sectors where cheap disks may perform
>    a partial write.  This would leave a corrupt journal.  If the last
>    bytes are a checksum then you can get some confidence that the entry
>    was fully written and is valid.

I will add a checksum mecanism.

Do you have any preferences regarding the checksum function ?

> 
>    Did I miss something?
> 
> 2. Byte-granularity means that read-modify-write is necessary to append
>    entries to the journal.  Therefore a failure could destroy previously
>    committed entries.

It's designed to be committed by 4KB blocks.

> 
>    Any ideas how existing journals handle this?
>