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Non 512 byte sectors

linux-lvm Phillip Susi 3 participants 6 messages Dec 13, 2012 – Dec 14, 2012

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Phillip Susi

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Is it possible to get dm to report > 512 byte sector size? If so, how?

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On 12/13/2012 10:23 AM, Phillip Susi expounded in part:

Quoted 6 lines

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Is it possible to get dm to report > 512 byte sector size? If so, how?

blockdev --getss /dev/vg/lv (or any other block device)

That is probably logical sector size, however. The only way I know to get physical sector size is:

hdparm -I /dev/sda

which reports logical and physical sector size. Some WD devices are known to lie about physical sector size, however.

If you always align stuff to 4096, there won't be a problem. In fact, new technology tends to have much larger alignment requirements for optimum performance - say 128k, the size of an erase block, or 64k for a RAID chunk size. So aligning everything to 1M should hold you for a while.

#2 lore raw
In reply to Phillip Susi
Quoted 4 lines · Phillip Susi

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Is it possible to get dm to report > 512 byte sector size? If so, how?

DM will automatically stack the physical and logical sector sizes based on the devices that are assembled into the DM device.

So if you create an LV on a 4K device the associated DM device will report the same physical and logical block size. You can verify as much by looking at:

/sys/block/dm-X/queue/physical_block_size
/sys/block/dm-X/queue/logical_block_size

There are more details about IO limits stacking here:
http://people.redhat.com/msnitzer/docs/io-limits.txt

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Quoted 10 lines · Mike Snitzer
Quoted 4 lines · Phillip Susi

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Is it possible to get dm to report > 512 byte sector size? If so, how?

DM will automatically stack the physical and logical sector sizes based on the devices that are assembled into the DM device.

Yes, but what if I am using two physical disks with 512 byte sectors, but want the dm to appear to have 2048 byte sectors?

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Quoted 9 lines · Phillip Susi

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Quoted 5 lines · Mike Snitzer
Quoted 2 lines · Phillip Susi

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Quoted 8 lines
Quoted 5 lines
Quoted 2 lines

Is it possible to get dm to report > 512 byte sector size? If so, how?

DM will automatically stack the physical and logical sector sizes based on the devices that are assembled into the DM device.

Yes, but what if I am using two physical disks with 512 byte sectors, but want the dm to appear to have 2048 byte sectors?

DM has no role in fabricating devices with artificial limits. What do you hope to accomplish with this?

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Quoted 2 lines · Mike Snitzer

DM has no role in fabricating devices with artificial limits. What do you hope to accomplish with this?

I am trying to fix dmraid on pdc arrays that are > 2TB. It seems that pdc has the virtual disk use a larger sector size so that you can partition it with MBR.

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