| Issue 5: | Block groups spanning over multiple devices. | |
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if i have three disks of details below disk A size 850 MB IDE disk B size 900MB sata disk C size 1000 MB pen drive I will join all the three disks using the LVM I will then format using ext2 FS and will have a single file system over the three devices ( i may be wrong here) Now suppose we have block group size as 100 MB then the block groups will be spread over the LVM disk the logically combined disk by LVM so the 8th block group(counting from 0) will have 50 MBs on disk A and 50 MBs Thus if the user says disk A will act as tier 1 disk B will act as tier 2 disk C will act as tier 3 and we will have a table of sort block groups 0 - 8 tier 1 block groups 9 - 17 tier 2 block groups 18 - 27 tier 3 then some files which have been allocated space in 8th block group of ext2 partition will get blocks in the tier 2 or disk B actually The problem here is : It may result in performance degradation The solution is : -> We can answer "the performance degradation is not significant as it will happen in atmost n block groups if the number of disks on the system is n -> This thing happens also in the case of the last block group of any ext2 partition not having size equal to multiple of block group size. So we can see the code and know how ext2 handles it and do it in our code also -> any other ideas ?
Dec 18, 2008
Project Member
#1
sandeepksinha
Summary:
Block groups spanning over multiple devices.
Dec 18, 2008
I think there could be a solution to it For Allocation we have a incore VFS inode or dirty inode we sink the inode onto disk For this we have to first find the Blockgroup then we find the inode in that Blockgroup where we have to sink the data. Inode would be in some sector in tier and then the associated sector can be checked that whether the sector lie in respected tier else we would allocate a new Blockgroup which would lie in that tier or we would provide error message stating couldn't allocate.
Dec 18, 2008
The status cannot be "accepted", as we have not concluded to any final decision.
Status:
Started
Dec 21, 2008
We need to take a final call on this asap ? Kindly give your opinion on each of these, we will face this issue tomorrow as well for rohit's model. It will be clearer by tomorrow that whether we will see this issue practically or not ? Rishi, you being the owner you have to keep track of this and have to take it to completion by coming tuesday.
Status:
In-progress
Dec 22, 2008
In regard to the above problem I think there exist two solution : 1. Either to leave the block group which is spanning over multiple devices and carry no block allocation for that block group. 2. Or we can allocate the block in that block group after checking two things firstly, the physical position (on disk) of the allocated block in the block group by the help of block number and secondly whether it is following the policy rule for that tier.
Dec 22, 2008
What do others have to say about this ? RKS, Rishi any insights ??
Dec 24, 2008
As per the phase I of ext2yaan we are just removing the disputed block groups from the list of block group range. We will work as if those block group numbers does not exist at all. An optimized and final solution will be thought upon in the optimization phase of the project.
Status:
WontFix
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