13.2. Brick Configuration

Procedure 13.1. Brick Configuration

1. LVM layer

   • Creating the Physical Volume
     The pvcreate command is used to create the physical volume. The Logical Volume Manager can use a portion of the physical volume for storing its metadata while the rest is used as the data portion.Align the I/O at the Logical Volume Manager (LVM) layer using --dataalignment option while creating the physical volume.
     The command is used in the following format:

     pvcreate --dataalignment alignment_value disk

     For JBOD, use an alignment value of 256K.
     In case of hardware RAID, the alignment_value should be obtained by multiplying the RAID stripe unit size with the number of data disks. If 12 disks are used in a RAID 6 configuration, the number of data disks is 10; on the other hand, if 12 disks are used in a RAID 10 configuration, the number of data disks is 6.
     For example, the following command is appropriate for 12 disks in a RAID 6 configuration with a stripe unit size of 128 KiB:

     # pvcreate --dataalignment 1280k disk

     The following command is appropriate for 12 disks in a RAID 10 configuration with a stripe unit size of 256 KiB:

     # pvcreate --dataalignment 1536k disk

     To view the previously configured physical volume settings for --dataalignment, run the following command:

     # pvs -o +pe_start disk
       PV         VG   Fmt  Attr PSize PFree 1st PE 
       /dev/sdb        lvm2 a--  9.09t 9.09t   1.25m

   • Creating the Volume Group
     The volume group is created using the vgcreate command.
     For hardware RAID, in order to ensure that logical volumes created in the volume group are aligned with the underlying RAID geometry, it is important to use the -- physicalextentsize option. Execute the vgcreate command in the following format:

     # vgcreate --physicalextentsize extent_size VOLGROUP physical_volume

     The extent_size should be obtained by multiplying the RAID stripe unit size with the number of data disks. If 12 disks are used in a RAID 6 configuration, the number of data disks is 10; on the other hand, if 12 disks are used in a RAID 10 configuration, the number of data disks is 6.
     For example, run the following command for RAID-6 storage with a stripe unit size of 128 KB, and 12 disks (10 data disks):

     # vgcreate --physicalextentsize 1280k VOLGROUP physical_volume

     In the case of JBOD, use the vgcreate command in the following format:

     # vgcreate VOLGROUP physical_volume

   • Creating the Thin Pool
     A thin pool provides a common pool of storage for thin logical volumes (LVs) and their snapshot volumes, if any.
     Execute the following command to create a thin-pool:

     # lvcreate --thinpool VOLGROUP/thin_pool --size <pool_size> --chunksize <chunk_size> --poolmetadatasize <meta_size> --zero n

     Recommended parameter values for thin pool creation

     poolmetadatasize

     Internally, a thin pool contains a separate metadata device that is used to track the (dynamically) allocated regions of the thin LVs and snapshots. The poolmetadatasize option in the above command refers to the size of the pool meta data device.

     The maximum possible size for a metadata LV is 16 GiB. Red Hat Gluster Storage recommends creating the metadata device of the maximum supported size. You can allocate less than the maximum if space is a concern, but in this case you should allocate a minimum of 0.5% of the pool size.

     chunksize

     An important parameter to be specified while creating a thin pool is the chunk size,which is the unit of allocation. For good performance, the chunk size for the thin pool and the parameters of the underlying hardware RAID storage should be chosen so that they work well together.

     For RAID-6 storage, the striping parameters should be chosen so that the full stripe size (stripe_unit size * number of data disks) is between 1 MiB and 2 MiB, preferably in the low end of the range. The thin pool chunk size should be chosen to match the RAID 6 full stripe size. Matching the chunk size to the full stripe size aligns thin pool allocations with RAID 6 stripes, which can lead to better performance. Limiting the chunk size to below 2 MiB helps reduce performance problems due to excessive copy-on-write when snapshots are used.

     For example, for RAID 6 with 12 disks (10 data disks), stripe unit size should be chosen as 128 KiB. This leads to a full stripe size of 1280 KiB (1.25 MiB). The thin pool should then be created with the chunk size of 1280 KiB.

     For RAID 10 storage, the preferred stripe unit size is 256 KiB. This can also serve as the thin pool chunk size. Note that RAID 10 is recommended when the workload has a large proportion of small file writes or random writes. In this case, a small thin pool chunk size is more appropriate, as it reduces copy-on-write overhead with snapshots.

     For JBOD, use a thin pool chunk size of 256 KiB.

     block zeroing

     By default, the newly provisioned chunks in a thin pool are zeroed to prevent data leaking between different block devices. In the case of Red Hat Gluster Storage, where data is accessed via a file system, this option can be turned off for better performance with the --zero n option. Note that n does not need to be replaced.

     The following example shows how to create the thin pool:

     lvcreate --thinpool VOLGROUP/thin_pool --size 800g --chunksize 1280k --poolmetadatasize 16G --zero n

   • Creating a Thin Logical Volume
     After the thin pool has been created as mentioned above, a thinly provisioned logical volume can be created in the thin pool to serve as storage for a brick of a Red Hat Gluster Storage volume.
     LVM allows multiple thinly-provisioned LVs to share a thin pool; this allows a common pool of physical storage to be used for multiple Red Hat Gluster Storage bricks and simplifies provisioning. However, such sharing of the thin pool metadata and data devices can impact performance in a number of ways.

     Note

     To avoid performance problems resulting from the sharing of the same thin pool, Red Hat Gluster Storage recommends that the LV for each Red Hat Gluster Storage brick have a dedicated thin pool of its own. As Red Hat Gluster Storage volume snapshots are created, snapshot LVs will get created and share the thin pool with the brick LV

     lvcreate --thin --name LV_name --virtualsize LV_size VOLGROUP/thin_pool

2. XFS Inode Size

   As Red Hat Gluster Storage makes extensive use of extended attributes, an XFS inode size of 512 bytes works better with Red Hat Gluster Storage than the default XFS inode size of 256 bytes. So, inode size for XFS must be set to 512 bytes while formatting the Red Hat Gluster Storage bricks. To set the inode size, you have to use -i size option with the mkfs.xfs command as shown in the following Logical Block Size for the Directory section.

3. XFS RAID Alignment

   When creating an XFS file system, you can explicitly specify the striping parameters of the underlying storage in the following format:

   mkfs.xfs other_options -d su=stripe_unit_size,sw=stripe_width_in_number_of_disks device

   For RAID 6, ensure that I/O is aligned at the file system layer by providing the striping parameters. For RAID 6 storage with 12 disks, if the recommendations above have been followed, the values must be as following:

   # mkfs.xfs other_options -d su=128k,sw=10 device

   For RAID 10 and JBOD, the -d su=<>,sw=<> option can be omitted. By default, XFS will use the thin-p chunk size and other parameters to make layout decisions.

4. Logical Block Size for the Directory

   An XFS file system allows to select a logical block size for the file system directory that is greater than the logical block size of the file system. Increasing the logical block size for the directories from the default 4 K, decreases the directory I/O, which in turn improves the performance of directory operations. To set the block size, you need to use -n size option with the mkfs.xfs command as shown in the following example output.
   Following is the example output of RAID 6 configuration along with inode and block size options:

   # mkfs.xfs -f -i size=512 -n size=8192 -d su=128k,sw=10 logical volume
   meta-data=/dev/mapper/gluster-brick1 isize=512    agcount=32, agsize=37748736 blks
            =    sectsz=512   attr=2, projid32bit=0
   data     =     bsize=4096   blocks=1207959552, imaxpct=5
            =    sunit=32     swidth=320 blks
   naming   = version 2   bsize=8192   ascii-ci=0
   log      =internal log   bsize=4096   blocks=521728, version=2
            =    sectsz=512   sunit=32 blks, lazy-count=1
   realtime =none    extsz=4096   blocks=0, rtextents=0

5. Allocation Strategy

   inode32 and inode64 are two most common allocation strategies for XFS. With inode32 allocation strategy, XFS places all the inodes in the first 1 TiB of disk. With larger disk, all the inodes would be stuck in first 1 TiB. inode32 allocation strategy is used by default.
   With inode64 mount option inodes would be replaced near to the data which would be minimize the disk seeks.
   To set the allocation strategy to inode64 when file system is being mounted, you need to use -o inode64 option with the mkfs.xfs command as shown in the following Access Time section.

6. Access Time

   If the application does not require to update the access time on files, than file system must always be mounted with noatime mount option. For example:

   # mount -t xfs -o inode64,noatime <logical volume> <mount point>

   This optimization improves performance of small-file reads by avoiding updates to the XFS inodes when files are read.

   /etc/fstab entry for option E + F
    <logical volume> <mount point>xfs     inode64,noatime   0 0

7. Performance tuning option in Red Hat Gluster Storage

   A tuned profile is designed to improve performance for a specific use case by tuning system parameters appropriately. Red Hat Gluster Storage includes tuned profiles tailored for its workloads. These profiles are available in both Red Hat Enterprise Linux 6 and Red Hat Enterprise Linux 7.

   Table 13.1. Recommended Profiles for Different Workloads

   Workload	Profile Name
   Large-file, sequential I/O workloads	rhgs-sequential-io
   Small-file workloads	rhgs-random-io
   Random I/O workloads	rhgs-random-io

   Earlier versions of Red Hat Gluster Storage on Red Hat Enterprise Linux 6 recommended tuned profiles rhs-high-throughput and rhs-virtualization. These profiles are still available on Red Hat Enterprise Linux 6. However, switching to the new profiles is recommended.
   To apply tunings contained in the tuned profile, run the following command after creating a Red Hat Gluster Storage volume.

   tuned-adm profile profile-name

   For example:

   tuned-adm profile rhgs-sequential-io

8. Writeback caching

   For small-file and random write performance, we strongly recommend writeback cache, that is, non-volatile random-access memory (NVRAM) in your storage controller. For example, normal Dell and HP storage controllers have it. Ensure that NVRAM is enabled, that is, the battery is working. Refer your hardware documentation for details on enabling NVRAM.
   Do not enable writeback caching in the disk drives, this is a policy where the disk drive considers the write is complete before the write actually made it to the magnetic media (platter). As a result, the disk write cache might lose its data during a power failure or even loss of metadata leading to file system corruption.

9. Allocation groups

   Each XFS file system is partitioned into regions called allocation groups. Allocation groups are similar to the block groups in ext3, but allocation groups are much larger than block groups and are used for scalability and parallelism rather than disk locality. The default allocation for an allocation group is 1 TiB.
   Allocation group count must be large enough to sustain the concurrent allocation workload. In most of the cases allocation group count chosen by mkfs.xfs command would give the optimal performance. Do not change the allocation group count chosen by mkfs.xfs, while formatting the file system.

10. Percentage of space allocation to inodes

    If the workload is very small files (average file size is less than 10 KB ), then it is recommended to set maxpct value to 10, while formatting the file system.