Good evening,
I understand that NTFS & VMDK do not relate to Solaris or ZFS, but I was
wondering if anyone has any experience of checking the alignment of data blocks
through that stack?
I have a VMware ESX 4.0 host using storage presented over NFS from ZFS
filesystems (recordsize 4KB). Within virtual machine VMDK files, I have
formatted NTFS filesystems, block size 4KB. Dedup is turned on. When I run ZDB
-DD, i see a figure of unique blocks which is higher than I expect, which makes
me wonder whether any given 4KB in the NTFS filesystem is perfectly aligned
with a 4KB block in ZFS?
e.g. consider two virtual machines sharing lots of the same blocks. Assuming
there /is/ a misalignment between NTFS & VMDK/VMDK & ZFS, if they're not in the
same order within NTFS, they don't align, and will actually produce different
blocks in ZFS:
VM1
NTFS 1---2---3---
AAAABBBBCCCC
AAAABBBBCCCC
ZFS 1---2---3---4---
ZFS blocks are " AA", "AABB" and so on ...
Then in another virtual machine, the blocks are in a different order:
VM2
NTFS 1---2---3---
CCCCAAAABBBB
CCCCAAAABBBB
ZFS 1---2---3---4---
ZFS blocks for this VM would be " CC", "CCAA", "AABB" etc. So, no overlap
between virtual machines, and no benefit from dedup.
I may have it wrong, and there are indeed 30,785,627 unique blocks in my setup,
but if there's a mechanism for checking alignment, I'd find that very helpful.
Thanks,
Chris
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