ZFS send/recv: weird stream size?

[Thomas Backman]

Hi all,
First off: not subscribed, please make sure I'm cc:ed or such.

I found a minor oddity today when running my backup script. It copies  
a pool into another, using "zfs send -R -I $LASTSNAP tank@$CURRSNAP |  
zfs recv -Fvd slave" (the variables should explain themselves).

Anyway, I got this in the middle:

receiving incremental stream of tank/var/crash at backup-20090626-1505  
into slave/var/crash at backup-20090626-1505
received 1.28GB stream in 51 seconds (25.6MB/sec)

tank/var/crash is a compressed filesystem, using lzjb.

Now, the only notable change in var/crash between backups was the  
addition of vmcore.22:

[root at chaos /var/crash]# du -sch *22*
  33K    core.txt.22
1.0K    info.22
  34M    kernel.debug.22
459M    vmcore.22
493M    total

[root at chaos /var/crash]# du -schA *22*
  88K    core.txt.22
512B    info.22
  57M    kernel.debug.22
2.2G    vmcore.22
2.3G    total

So, if the new files' compressed size is 493MB, and the real size is  
2.3GB, how come zfs send/recv transfers 1.28 GB?

Other, possibly interesting (although I doubt it) stats:

[root at chaos ~]# zfs get compress,compressratio tank/var/crash
NAME            PROPERTY       VALUE           SOURCE
tank/var/crash  compression    lzjb            local
tank/var/crash  compressratio  2.90x           -

[root at chaos ~]# du -shc /var/crash
1.0G    /var/crash
1.0G    total
[root at chaos ~]# du -shcA /var/crash
6.9G    /var/crash
6.9G    total

Regards,
Thomas