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Old 15th November 2011, 09:38 PM
convict Offline
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Does F16 installer align partitions correctly (if necessary)?

If I do a custom partition layout in F16, are they automatically aligned on any relevant boundaries? Or, for that matter, is proper alignment even necessary on a single HD setup? I've read several articles but all of them seem to in some way be talking about either SSDs or RAID setups, so now I'm a bit confused as to whether or not I should be worrying about this at all. But if it is relevant and F16 installer doesn't align them properly, how would I go about creating a properly aligned partition scheme outside of Fedora installed?
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Old 15th November 2011, 10:03 PM
PiElle Offline
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Re: Does F16 installer align partitions correctly (if necessary)?

In my experience Anaconda sets the partitions properly ...
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Old 16th November 2011, 01:02 AM
srs5694 Offline
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Re: Does F16 installer align partitions correctly (if necessary)?

To elaborate on when it's required, to the best of my knowledge, three technologies require attention to alignment:
  • Advanced Format hard disks -- those that use 4096-byte physical sectors but that report a 512-byte sector size to the OS
  • Some (but not all) types of RAID configuration
  • SSDs

If you don't use any of these technologies, you don't need to worry about alignment. If your partitions are aligned as described below, you'll lose a tiny amount of disk space, but nothing worse will happen.

Details of the minimum alignment values vary between these technologies, as do the precise effects of misalignment. (I wrote an article on Advanced Format disks for IBM developerWorks a while back, with some simple benchmark tests on an early AF disk.) To the best of my knowledge, though, aligning on 1 MiB (2048-sector) boundaries produces correct alignment with all of these technologies. Most partitioning tools released in the last year or so align on 1 MiB boundaries by default, so you're pretty safe with modern tools. FWIW, Microsoft switched to 1 MiB alignment with Windows Vista, but Linux tools lagged behind on this change. If you're using something older than a year or two, there's a chance that it aligns on cylinder boundaries. Such alignment was beneficial 20 or more years ago, but has been pointless for at least the past decade, and will likely degrade performance if you use AF disks, some types of RAID, or SSDs.

As PiElle says, recent versions of Anaconda have aligned properly (on 1 MiB boundaries). I'm not sure which version of Fedora was the first to have an Anaconda that aligns on 1 MiB boundaries, though.

If there's any doubt in your mind, the best approach is to check the alignment. You can do this with parted, but you've got to tell it to report values in sectors, via "unit s", as in:

# parted /dev/sdb unit s print
Model: ATA MAXTOR STM332062 (scsi)
Disk /dev/sdb: 625142448s
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start    End         Size        File system  Name         Flags
 1      2048s    514047s     512000s     fat32        EFI System   boot, hidden
 2      514048s  923647s     409600s     ext2         Spare /boot
 3      923648s  625142414s  624218767s               Linux LVM    lvm
This example shows start sector numbers of 2048, 514,048, and 923,648. These values are all multiples of 2048, meaning that the partitions are all properly aligned. In some cases, alignment on smaller values is fine; on an Advanced Format disk, if the start sectors are multiples of 8, all is fine.

Even if a partition is not aligned on a suitable multiple, it might not be a problem. Extended partitions on MBR disks need not be so aligned, for instance -- it's mainly filesystem data structures that need to be aligned, and they're stored in primary partitions, logical partitions, or logical volumes, not in extended partitions. BIOS Boot Partitions on GPT disks also don't need to be aligned. (In theory, they do suffer performance degradation, but probably only on writing. BIOS Boot Partitions are so small and are written so seldom that improper alignment has no effect on day-to-day use of the computer.)
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Old 16th November 2011, 08:46 AM
convict Offline
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Re: Does F16 installer align partitions correctly (if necessary)?

Thanks, that's brilliant - I did end up partitioning manually using gparted though (installed before I got a reply), having it align on MiB boundaries, and apparently that did produce the right result according to parted. All's well now.
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Old 17th November 2011, 05:05 PM
PiElle Offline
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Re: Does F16 installer align partitions correctly (if necessary)?

@ srs5694.
Very good explanation of the problem!
I have an Advanced Format hard disk and both Anaconda and Gparted (latest versions) set the partitions properly!
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