{"id":302,"date":"2010-09-04T03:50:24","date_gmt":"2010-09-04T03:50:24","guid":{"rendered":"http:\/\/poojanwagh.opalstacked.com\/techblog\/un\/partitioning-with-gpt\/"},"modified":"2010-09-04T03:54:11","modified_gmt":"2010-09-04T03:54:11","slug":"partitioning-with-gpt","status":"publish","type":"post","link":"https:\/\/tech.poojanblog.com\/blog\/un\/partitioning-with-gpt\/","title":{"rendered":"Partitioning with gpt"},"content":{"rendered":"

Sometime ago, I setup a ZFS system using glabel on FreeBSD. I recently stumbled across this FreeBSD diary entry<\/a>, which describes to do almost the same thing using gpt. I like this method better, because it results in a partition that is on a 4kB boundary, which works better for some hard drives (whose sector size is 4kB).<\/p>\n

Here\u2019s what I did:<\/p>\n<\/p>\n

#gpart show da0
\n=>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 63\u00a0 1953525105\u00a0 da0\u00a0 MBR\u00a0 (932G)
\n63\u00a0 1953525105\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 – free –\u00a0 (932G)\u00a0 <\/code>\n

OK: that’s the starting point. I have a MBR-style partition which is set up as one 932G partition and some free space.<\/p>\n

I wasn’t sure exactly how to get rid of it, so I tried:<\/p>\n<\/p>\n

# gpart delete -i 1 da0
\nda0s1 deleted<\/p>\n

<\/code>\n

I still have a MBR-style partition table. Next, as I read in the FreeBSD diary, I need to figure out how many sectors I have:<\/p>\n<\/p>\n

# dmesg | grep “da0”
\nda0 at umass-sim0 bus 0 scbus0 target 0 lun 0
\nda0: <Hitachi HDS721010CLA332 > Fixed Direct Access SCSI-2 device
\nda0: 40.000MB\/s transfers
\nda0: 953869MB (1953525168 512 byte sectors: 255H 63S\/T 121601C)<\/p>\n

<\/code>\n

OK: 1953525168 sectors of 512 bytes each. However, for some reason, that isn’t what gpt shows:<\/p>\n
\n# gpart show da0
\n=> 34 1953525101 da0 GPT (932G)
\n 34 1953525101 – free – (932G)
\n<\/code>\n

I’m not sure if this smaller size is because 34 sectors are taken up by the partition table, or if it’s a cylinder-boundary thing.<\/p>\n

As the article recommends, I want to leave the first 1MB free (Windows does this, too). I also want to leave around 200MB free at the end of the disk (to allow for some slack in case I want to replace this disk). So, I have 1953525168 sectors (total) – 2048 sectors (1MB) – 409600 (200MB) = 1953113453 sectors:<\/p>\n


\nfbsd2# gpart add -b 2048 -s 1953113453 -t freebsd-zfs -l Hitachi_1TB da0
\nda0p1 added
\n<\/code>
\n
\n# gpart show da0
\n=> 34 1953525101 da0 GPT (932G)
\n 34 2014 – free – (1.0M)
\n 2048 1953113453 1 freebsd-zfs (931G)
\n 1953115501 409634 – free – (200M)
\n<\/code><\/p>\n

Looks good: the partition is 931GB large. I have 200MB of slack at the end, and it starts 1MB into the disk. Now, let’s replace the old glabel-based partition with this one:<\/p>\n


\n# zpool replace tank label\/hitachi_1tb gpt\/Hitachi_1TB
\n<\/code>
\n
\n# zpool status
\n pool: tank
\n state: DEGRADED
\nstatus: One or more devices is currently being resilvered. The pool will
\n continue to function, possibly in a degraded state.
\naction: Wait for the resilver to complete.
\n scrub: resilver in progress for 0h0m, 0.01% done, 95h54m to go
\nconfig:<\/p>\n

NAME STATE READ WRITE CKSUM
\n tank DEGRADED 0 0 0
\n raidz1 DEGRADED 0 0 0
\n label\/fantom_500gb ONLINE 0 0 0 6.27M resilvered
\n replacing DEGRADED 0 0 0
\n label\/hitachi_1tb OFFLINE 0 1.83K 0
\n gpt\/Hitachi_1TB ONLINE 0 0 0 9.41M resilvered
\n label\/fantom_1tb ONLINE 0 0 0 6.25M resilvered
\n<\/code><\/p>\n

Pretty cool.<\/p>\n