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Thread: GPT Partitions Limitation (Not partition size)

  1. #1
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    GPT Partitions Limitation (Not partition size)

    Hello.

    I am currently finishing up a project for my company using Ubuntu Server 10.10 on dell r610s connected to Promise Technologies SAS storage solutions.

    I have two arrays that are 42.0TB large (/dev/sdb, /dev/sdc). I am using parted to make 2TB partitions that will be used with enterprise open ISCSI and logged in by our VMware infrastructure ESX hosts to create a VMFS extentable volume (large stitched up datastore).

    On one of the arrays, I have successfully parted 10 x 2TB partitions to the system, and they show up in /dev as device nodes (sdb1, sdb2, sdb3, etc). ESX has 20TB at its disposal (yay!)

    Today, I carved the remaining space on the first array and had a total of 21 x ~2TB partitions. I read somewhere that GPT supports up to 128 total partitions in its table. For me, however, it seems the limitation is 16 partitions. All of my parted partitions above 16 do not have /dev device nodes (sdb17, sdb18, etc).


    Since I had another array at my disposal, I carved its entire 42TB of available space into 2TB chunks like I did with the first array. I took a look at the device nodes, and the same thing happened: first 16 partitions show up as device nodes, the remaining are nowhere to be seen.

    I have multiple resolution points for this, none of which keep the array at 42TBs. I am wondering if anybody else has experienced this "limitation", or if it is true that Ubuntu can only do 16 partitions per device?
    Last edited by revresxunil; December 14th, 2010 at 03:01 AM.

  2. #2
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    Re: GPT Partitions Limitation (Not partition size)

    I put gpt on my 160GB drive just to learn about it. So I cannot help much on your size drives.

    Did you use gdisk?

    GPT fdisk Tutorial -srs5694 in forums
    http://ubuntuforums.org/showthread.php?t=1439794
    http://www.rodsbooks.com/gdisk/
    For info on UEFI boot install & repair:
    http://ubuntuforums.org/showthread.php?t=2147295
    Please use Thread Tools above first post to close thread when/if answered completely.




  3. #3
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    Re: GPT Partitions Limitation (Not partition size)

    gdisk shows all 21 partitions correctly



    I have not yet partitioned with gdisk to see if it might write the GPT partition table differently. I will give that a try.

  4. #4
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    Re: GPT Partitions Limitation (Not partition size)

    I only respond to PM's in threads, but you might politely ask srs5694 for assistance. He is often on this forum and I think he looks for those with issues on gpt. He may pop in some anyway since your title has gpt in it.
    For info on UEFI boot install & repair:
    http://ubuntuforums.org/showthread.php?t=2147295
    Please use Thread Tools above first post to close thread when/if answered completely.




  5. #5
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    Re: GPT Partitions Limitation (Not partition size)

    I erased my GPT table and recreated it with gdisk. gdisk is much more friendly in terms of partitioning, but still results in 16 of the 20 partitions showing up as device nodes.

    Since I have not heard anyone else experiencing this problem, I'm going to resolve it by breaking the array in half to avoid the sweet 16.

  6. #6
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    Re: GPT Partitions Limitation (Not partition size)

    GPT can use huge partitions, do not worry about it.

    If you have a 4U machine with 16 bays, using 2 TB disks you get 32 TB of raw space and 28 TB of RAID 6 storage.

    Bigger disks are coming or are available for $$$$

    You could have a 28 TB partition with ext4 and share it with SAMBA for Windows clients or NFS for other clients.
    SERVER: ATX chassis, 460W, M4A77D, Athlon64 X2 4200+ 65W, 8 GB DDR2, Server 2012 R2 + Hyper-V 4.0

  7. #7
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    Re: GPT Partitions Limitation (Not partition size)

    Unfortunately for me, I am creating a Vmware VMFS datastore, which is limited to 2TB extents. Therefore, I must limit my partition size to 2TB - 512 bytes.

  8. #8
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    Re: GPT Partitions Limitation (Not partition size)

    I just did a test, and I created 22 500 MiB partitions on a USB flash drive. They all show up fine under both Ubuntu 10.10 and Gentoo:

    Code:
    ls /dev/sdb*
    /dev/sdb    /dev/sdb12  /dev/sdb16  /dev/sdb2   /dev/sdb3  /dev/sdb7
    /dev/sdb1   /dev/sdb13  /dev/sdb17  /dev/sdb20  /dev/sdb4  /dev/sdb8
    /dev/sdb10  /dev/sdb14  /dev/sdb18  /dev/sdb21  /dev/sdb5  /dev/sdb9
    /dev/sdb11  /dev/sdb15  /dev/sdb19  /dev/sdb22  /dev/sdb6
    I'm not sure why you're having problems, but my suspicion is it's something to do with the way udev is creating device nodes. Perhaps there's a bug that limits the number of partition nodes that udev creates for "real" hard disks, vs. USB flash drives. (As far as Linux is concerned, your hardware RAID setup is just a regular hard disk.) Certainly it's not a GPT issue per se. You're correct that GPT supports up to 128 partitions by default. That default value can be changed, although it's only "legal" to increase it. (It can be decreased by GPT fdisk, and this seems to work fine with every OS I've tried, but it violates the specifications. Increasing the limit is fine, although some versions of libparted-based tools flake out with such setups.) In any event, if the limit were decreased to 16 partitions in the partition table structures, you wouldn't be able to create more than 16 partitions with gdisk (or anything else), so that's definitely not the source of the problem.

  9. #9
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    Re: GPT Partitions Limitation (Not partition size)

    srs5694,

    Thanks for the reply. I appreciate your time with the test, as it provides a piece to my puzzle.

    I am now under the assumption that the problem lies in the kernel, at which point I would rather break up the array, than make any changes to the stable kernel I'm using.

    The fact that I can create more than 16 partitions using parted and gdisk shows clearly its not a GPT problem.

  10. #10
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    Re: GPT Partitions Limitation (Not partition size)

    I wouldn't blame the kernel. Offhand, I don't know of any kernel options that affect this issue, and my own tests with Ubuntu used the distribution's standard kernel, while my Gentoo test was with a stock kernel (from kernel.org) that I compiled myself, so unless you've recompiled your kernel and messed with an option I don't know about, it's not likely to be the kernel.

    In modern Linux systems, device nodes are created by the udev utility, so that's the likely source of the problem. Given that we both used Ubuntu 10.10 systems (albeit probably slightly different variants), my guess is that there's a bug in Ubuntu's udev configuration that only manifests for internal drives, or perhaps only when a drive is detected during the boot process (vs. later on).

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