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June 27th, 2013 01:00
Snapsure operations
I cannot understand part of the operations in Snapsure, referring to P.96 and 97 from VNX local protection foundation doc.
Reading from an older Snap/ckpt
How come when trying to read from the older ckpt, the new blockmap is consulted as well as the new bitmap?!!
Once SnapSure arrives at the newest snapshot, the bitmap can then be utilized to determine any other blocks that have changed in the PFS. In the example above, the bitmap says that the first, second, and sixth blocks have changed in the PFS. Notice that we already have the second block in the PFS accounted for with a “B” from the previous slide due to blockmap1. If SnapSure finds blocks that already have been accounted for, it will simply skip it and go to the next block that is represented in the bitmap.
Once SnapSure arrives at the newest snapshot, the bitmap can then be utilized to determine any other blocks that have changed in the PFS. In the example above, the bitmap says that the first, second, and sixth blocks have changed in the PFS. Notice that we already have the second block in the PFS accounted for with a “B” from the previous slide due to blockmap1. If SnapSure finds blocks that already have been accounted for, it will simply skip it and go to the next block that is represented in the bitmap.
christopher_ime
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July 1st, 2013 00:00
Shawky,
To understand this you have to remember, with CoFW: PFS *always* has the latest updates. In this case, when Ckpt 2 was created, there were changes made to PFS and would not be reflected in the Blockmap for Ckpt 1.
So let's start off by only reading Blockmap1 and see what we get back:
PFS (Production File System)
===
J L C D K S
Reviewing Blockmap1 (Ckpt 1):
=======================
a) From the diagram above, this tells us that PFS Block 2 and Block 5 were modified so the original data (preserving the PIT) needs to be retrieved from SavVol Block 1 and Block 2, respectively. What may be helpful also is rereading the section prior to this which I assume walks you through the workflow when writes are made to a PFS with active checkpoint(s).
b) That would result in:
J B C D E S
c) However, as we know Ckpt 1 is:
A B C D E F
So we are close, but Blocks 1 and 6 are unaccounted for.
Retrieving from Blockmap2:
=====================
When Ckpt 2 was created, PFS Blocks 1, 2, and 6 were changed as reflected by Blockmap2 (and in this case the Bitmap which only reflects just the latest checkpoint). Therefore, we need to retrieve the original PFS data which was relocated to SavVol. However, since Block 2 is already accounted for (from Ckpt 1's perspective we don't care that it changed again), we just need to retrieve the original PFS Block 1 and Block 6 which were saved to SavVol Block 3 and Block 5, respectively, which results in:
A B C D E F
Does this help?