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| author | Richard W.M. Jones <rjones@redhat.com> | 2012-05-14 17:47:57 +0100 |
|---|---|---|
| committer | Richard W.M. Jones <rjones@redhat.com> | 2012-05-18 14:11:10 +0100 |
| commit | 6b6c372d5d81699439e00b450561b7012b79929c (patch) | |
| tree | 3901c709cb68eebabb8c37d31a9d6d3a5738f6c9 | |
| parent | 356f6fd6b31c3da5efbddb72458946c21531d004 (diff) | |
| download | libguestfs-6b6c372d5d81699439e00b450561b7012b79929c.tar.gz libguestfs-6b6c372d5d81699439e00b450561b7012b79929c.tar.xz libguestfs-6b6c372d5d81699439e00b450561b7012b79929c.zip | |
doc: Add notes on how GPT works on 4k sector disks.
I used scsi_debug to create a 4k sector virtual disk:
modprobe scsi_debug dev_size_mb=128 sector_size=4096
I then used 'gdisk' to create lots of partitions, and used 'hexdump'
to examine what was written to disk.
(cherry picked from commit 2fd2c85f6a42575749b9d38bd2f1512ae25db760)
| -rw-r--r-- | daemon/parted.c | 4 | ||||
| -rw-r--r-- | resize/resize.ml | 15 |
2 files changed, 16 insertions, 3 deletions
diff --git a/daemon/parted.c b/daemon/parted.c index 26100422..1a020f9b 100644 --- a/daemon/parted.c +++ b/daemon/parted.c @@ -199,7 +199,9 @@ do_part_disk (const char *device, const char *parttype) * * - aligned operations are faster * - absolute minimum recommended alignment is 64K (1M would be better) - * - GPT requires at least 34 sectors at the end of the disk. + * - GPT requires at least 34 sectors* at the end of the disk. + * + * *=except for 4k sector disks, where only 6 sectors are required */ const char *startstr = "128s"; const char *endstr = "-128s"; diff --git a/resize/resize.ml b/resize/resize.ml index 81db30fd..af673266 100644 --- a/resize/resize.ml +++ b/resize/resize.ml @@ -864,8 +864,19 @@ let () = let start = if parttype <> GPT then 512L else - (* XXX With 4K sectors does GPT just fit more entries in a - * sector, or does it always use 34 sectors? + (* With 512 byte sectors, GPT looks like: + * 512 bytes sector 0 protective MBR + * 1024 bytes sector 1 GPT header + * 17408 bytes sectors 2-33 GPT entries (up to 128 x 128 bytes) + * + * With 4K sectors, GPT puts more entries in each sector, so + * the partition table looks like this: + * 4096 bytes sector 0 protective MBR + * 8192 bytes sector 1 GPT header + * 24576 bytes sectors 2-5 GPT entries (up to 128 x 128 bytes) + * + * qemu doesn't support 4k sectors yet, so let's just use the + * 512 sector number for now. *) 17408L in |