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Linux Recommended Settings

Applies to:  FlashArray

To ensure the best performance with the Pure Storage FlashArrays, please use this guide for configuration and implementation of Linux hosts in your environment.  These recommendations apply to the versions of Linux that we have certified as per our Compatibility Matrix.

Boot from SAN Considerations

If you are using a LUN to boot from SAN, you need to ensure the changes in your configuration files are applied upon rebooting. This is done by rebuilding the initial ramdisk (initrd or initramfs) to include the proper kernel modules, files and configuration directives after the configuration changes have been made. As the procedure slightly varies depending on the host, we recommend that you refer to your vendor's documentation for the proper procedure. 

When rebuilding the initial ramdisk, you need to confirm that the necessary dependencies are in place before rebooting the host to avoid any errors during boot. Refer to your vendor's documentation for specific commands to confirm this information.

 

An example command from Oracle Linux to check the initramfs:

lsinitrd /boot/initramfs-$(uname -r).img | grep dm

An example file that may be missing that could result in failure to boot:

...(kernel build)/kernel/drivers/md/dm-round-robin.ko

HBA I/O Timeout Settings

Though the Pure Storage FlashArray is designed to service IO with consistent low latency, there are error conditions that can cause much longer latencies and it is therefore important to ensure dependent servers and applications are tuned appropriately to ride out these error conditions without issue. By design, given the worst case, recoverable error condition, the FlashArray will take up to 60 seconds to service an individual IO.  You can do this with the following commands. 

You can check current timeout settings using the following command as root

find /sys/class/scsi_generic/*/device/timeout -exec grep -H . '{}' \;

For versions below RHEL 6, you can add the following command(s) into rc.local 

echo 60 > /sys/block/<Dev_name>/device/timeout
Note that the default timeout for normal file system commands is 60 seconds when udev is being used. If udev is not in use, the default timeout is 30 seconds.  If you are running RHEL 6+, and want to ensure the rules persist, then use the udev method documented below

Queue Settings

We recommend two changes to the queue settings.  The first selects the 'noop' I/O scheduler, which has been shown to get better performance with lower CPU overhead than the default schedulers (usually 'deadline' or 'cfq').  The second change eliminates the collection of entropy for the kernel random number generator, which has high cpu overhead when enabled for devices supporting high IOPS.

Manually Changing Queue Settings 

(not required unless LUNs are already in use with wrong settings)

These settings can be safely changed on a running system, by locating the Pure LUNs:

grep PURE /sys/block/sd*/device/vendor

And writing the desired values into sysfs files:

echo noop > /sys/block/sdx/queue/scheduler

An example for loop is shown here to quickly set all Pure luns to the desired 'noop' elevator.

for disk in $(lsscsi | grep PURE | awk '{print $6}'); do
    echo noop > /sys/block/${disk##/dev/}/queue/scheduler
done

All changes in this section take effect immediately, without rebooting for RHEL5 and 6. RHEL 4 releases will require a reboot.

Applying Queue Settings with udev

Once the IO scheduler elevator has been set to 'noop' it is often desired to keep the setting persistent, after reboots. 

Step 1: Create the Rules File

Create a new file in the following location (for each respective OS). The Linux OS will use the udev rules to set the elevators after each reboot.

RHEL:
/etc/udev/rules.d/99-pure-storage.rules
Ubuntu:  
/lib/udev/rules.d/99-pure-storage.rules

Step 2: Add the Following Entries to the Rules File  (Version Dependent)

The following entries automatically sets the elevator to 'noop' each time the system is rebooted. Create a file that has the following entries, ensuring each entry exists on one line with no carriage returns:

For RHEL 6.x, 7.x and SuSE
# Recommended settings for Pure Storage FlashArray.

# Use noop scheduler for high-performance solid-state storage
ACTION=="add|change", KERNEL=="sd*[!0-9]", SUBSYSTEM=="block", ENV{ID_VENDOR}=="PURE", ATTR{queue/scheduler}="noop"

# Reduce CPU overhead due to entropy collection
ACTION=="add|change", KERNEL=="sd*[!0-9]", SUBSYSTEM=="block", ENV{ID_VENDOR}=="PURE", ATTR{queue/add_random}="0"

# Spread CPU load by redirecting completions to originating CPU
ACTION=="add|change", KERNEL=="sd*[!0-9]", SUBSYSTEM=="block", ENV{ID_VENDOR}=="PURE", ATTR{queue/rq_affinity}="2"

# Set the HBA timeout to 60 seconds
ACTION=="add", SUBSYSTEMS=="scsi", ATTRS{model}=="FlashArray      ", RUN+="/bin/sh -c 'echo 60 > /sys/$DEVPATH/device/timeout'"

Please note that 6 spaces are needed after "FlashArray" under "Set the HBA timeout to 60 seconds" above for the rule to take effect.

For RHEL 5.x
# Recommended settings for Pure Storage FlashArray.
 
# Use noop scheduler for high-performance solid-state storage
ACTION=="add|change", KERNEL=="sd*[!0-9]|", SYSFS{vendor}=="PURE*", RUN+="/bin/sh -c 'echo noop > /sys/$devpath/queue/scheduler'" 

It is expected behavior that you only see the settings take effect for the sd* devices.  The dm-* devices will not reflect the change directly but will inherit it from the sd* devices that make up it's path. 

Maximum IO Size Settings

The maximum allowed size of an I/O request in kilobytes is determined by the max_sectors_kbsetting in sysfs. This restricts the largest IO size that the OS will issue to a block device The Pure Storage FlashArray can handle a maximum of 4MB writes. Therefore, we need to make sure that the maximum allowed IO size matches our expectations. You can check your current settings to determine what the IO size is, and as long as it does not exceed 4096, you should be fine.

Verify the Current Setting

  1. Check which block device you are using with the Pure Storage array.
    If you know which device you're looking at already (dm-#)
    [root@host ~]# multipath -ll | grep -A 7 -B 0 "dm-6"
    3624a9370ffa9a01386b3410600011036 dm-6 PURE,FlashArray
    size=35G features='0' hwhandler='0' wp=rw
    `-+- policy='queue-length 0' prio=1 status=active
      |- 1:0:0:9 sdf 8:80   active ready running
      |- 0:0:1:9 sdx 65:112 active ready running
      |- 1:0:1:9 sdl 8:176  active ready running
      `- 0:0:0:9 sdr 65:16  active ready running
    

    OR

    If we want to know all PURE volumes presented to the host

    multipath -ll | grep -i PURE
    
  2. Check the "max_sectors_kb" on your Linux host ( regardless of the kernel version or Linux distribution). Customer will need to know which device.
    $ cat /sys/block/sda/queue/max_sectors_kb
    512
    

If the value is  ≤ 4096, then no action is necessary.  However, if this value is > 4096, we recommend that you change the max to 4096. 

Changing the Maximum Value 

Reboot Persistent

We recommend that you add the value to your UDEV rules file (99-pure-storage.rules) created above.  This will ensure that the setting persists through a reboot.  To change that value please do the following: 

  1. Changing the "max_sectors_kb" value by adding it to the UDEV rules (Reboot Persistent):adding it to the UDEV rules)
    echo 'ACTION=="add|change", KERNEL=="sd*[!0-9]", SUBSYSTEM=="block", ENV{ID_VENDOR}=="PURE", ATTR{queue/max_sectors_kb}="4096"' >> /etc/udev/rules.d/99-pure-storage.rules

     NOTE: The location of your rules file may be different depending on your OS version, so please double check the command before running it. 

  2. Reboot the host. 
  3. Check the value again.
Immediate Change but Won't Persist Through Reboot

This command should only be run if you are sure there are no running services depending on that volume, otherwise you can risk an application crash.

If you need to make the change immediately, but cannot wait for a maintenance window to reboot, you can also change the setting with the following command: 

echo  %VALUE% > /sys/block/sdz/queue/max_sectors_kb

%VALUE% should be ≤ 4096

Recommended DM-Multipathd Settings

ActiveCluster: Additional multipath settings are required for ActiveCluster. Please see ActiveCluster Requirements and Best Practices.

The Multipath Policy defines how the host distributes IOs across the available paths to the storage. The Round Robin (RR) policy distributes IOs evenly across all Active/Optimized paths. A newer MPIO policy, queue-length, is similar to round robin in that IOs are distributed across all available Active/Optimized paths, however it provides some additional benefits. The queue-length path selector will bias IOs towards paths that are servicing IO quicker (paths with shorter queues). In the event that one path becomes intermittently disruptive or is experiencing higher latency, queue-length will prevent the utilization of that path reducing the effect of the problem path.

The following are recommended entries to existing multipath.conf files (/etc/multipath.conf) for Linux OSes.  Add the following to existing section for controlling Pure devices.

Please note that fast_io_fail_tmo and dev_loss_tmo do not apply to iSCSI.

RHEL 7.3+
No manual changes required. The dm-multipath config shown below for PURE is default with the device-mapper version included in RHEL / Oracle Linux 7.3+
  device {
        vendor "PURE"
        product "FlashArray"
        path_grouping_policy "multibus"
        path_selector "queue-length 0"
        path_checker "tur"
        features "0"
        hardware_handler "0"
        prio "const"
        failback immediate
        fast_io_fail_tmo 10
        dev_loss_tmo 60
        user_friendly_names no
    }
}

Included in RHEL 7.3+ is device-mapper-multipath-0.4.9-99
Support added for PURE FlashArray - With this release, multipath has added built-in configuration support for the PURE FlashArray (BZ#1300415)

Supporting Info:
RHEL 6.2+, SLES 12, and supporting kernels
defaults {
   polling_interval      10
   find_multipaths       yes
}
devices {
   device {
       vendor                "PURE"
       path_selector         "queue-length 0"
       path_grouping_policy  group_by_prio
       path_checker          tur
       fast_io_fail_tmo      10
       dev_loss_tmo          60
       no_path_retry         0
       hardware_handler      "1 alua"
       prio                  alua
       failback              immediate
   }
}
RHEL 5.7+ - 6.1 and supporting kernels
defaults {
    polling_interval      10
}
 
devices {
    device {
        vendor                "PURE"
        path_selector         "round-robin 0"
        path_grouping_policy  multibus
        rr_min_io             1
        path_checker          tur
        fast_io_fail_tmo      10
        dev_loss_tmo          60
        no_path_retry         0
    }
}
RHEL 5.6 and below, and supporting kernels
defaults {
polling_interval 10
}

devices {

        device {
               vendor                "PURE"
               path_selector         "round-robin 0"
               path_grouping_policy  multibus
               rr_min_io             1
               path_checker          tur
               no_path_retry         0
               }
        }
Oracle VM Server
device {
                vendor                "PURE"
                product               "FlashArray"
                path_selector         "queue-length 0"
                path_grouping_policy  group_by_prio
                path_checker          tur
                fast_io_fail_tmo      10
                dev_loss_tmo          60
                no_path_retry         0
                hardware_handler      "1 alua"
                prio                  alua
                failback              immediate
                user_friendly_names   no
        }

More information on multipath settings can be found here: RHEL Documentation

Verifying DM-Multipathd Configuration

You can check the setup by looking at "multipath -ll".  

6.2+ (queue-length)

# multipath -ll

Correct Configuration:
mpathe (3624a93709d5c252c73214d5c00011014) dm-2 PURE,FlashArray
size=100G features='0' hwhandler='0' wp=rw
`-+- policy='queue-length 0' prio=1 status=active
  |- 1:0:0:4  sdd  8:48   active ready running
  |- 1:0:1:4  sdp  8:240  active ready running
  |- 1:0:2:4  sdab 65:176 active ready running
  |- 1:0:3:4  sdan 66:112 active ready running
  |- 2:0:0:4  sdaz 67:48  active ready running
  |- 2:0:1:4  sdbl 67:240 active ready running
  |- 2:0:2:4  sdbx 68:176 active ready running
  `- 2:0:3:4  sdcj 69:112 active ready running
...
 
Incorrect Configuration (check for unecessary spaces in multipath.conf):

3624a9370f35b420ae1982ae200012080 dm-0 PURE,FlashArray
size=500G features='0' hwhandler='0' wp=rw
 |-+- policy='round-robin 0' prio=0 status=active
 | `- 2:0:0:3 sdc 8:32 active undef running
 |-+- policy='round-robin 0' prio=0 status=enabled
 | `- 3:0:0:3 sdg 8:96 active undef running
 |-+- policy='round-robin 0' prio=0 status=enabled
 | `- 1:0:0:3 sdaa 65:160 active undef running
 `-+- policy='round-robin 0' prio=0 status=enabled
 `- 0:0:0:3 sdak 66:64 active undef running
 ...

Below 6.2 (Round Robin)

    # multipath -ll
    ...
    Correct Configuration:
    3624a9370f35b420ae1982ae200012080 dm-0 PURE,FlashArray
    size=500G features='0' hwhandler='0' wp=rw
    `-+- policy='round-robin 0' prio=0 status=active
    |- 2:0:0:3 sdc 8:32 active undef running
    |- 3:0:0:3 sdg 8:96 active undef running
    |- 1:0:0:3 sdaa 65:160 active undef running
    `- 0:0:0:3 sdak 66:64 active undef running
     
    ...
    Incorrect Configuration (check for unecessary spaces in multipath.conf):

    3624a9370f35b420ae1982ae200012080 dm-0 PURE,FlashArray
    size=500G features='0' hwhandler='0' wp=rw
    |-+- policy='round-robin 0' prio=0 status=active
    | `- 2:0:0:3 sdc 8:32 active undef running
    |-+- policy='round-robin 0' prio=0 status=enabled
    | `- 3:0:0:3 sdg 8:96 active undef running
    |-+- policy='round-robin 0' prio=0 status=enabled
    | `- 1:0:0:3 sdaa 65:160 active undef running
    `-+- policy='round-robin 0' prio=0 status=enabled
    `- 0:0:0:3 sdak 66:64 active undef running
    ...

Excluding Third-Party vendor LUNs from DM-Multipathd

When systems have co-existing multipathing software, it is often desired to exclude control from one multipathing software in order to allow control by another multipathing software. 

The following is an example on using DM-Multipathd to blacklist LUNs from a third party vendor. The syntax blocks DM-Multipathd from controlling those luns that are "blacklisted".

The following can be added to the 'blacklist' section of the multipath.conf file.

blacklist {
                device {
                             vendor "XYZ.*"
                             product ".*"
                             }

                device {
                             vendor "ABC.*"
                             product ".*"
                             }
                 }

device-mapper-multipath and EMC PowerPath

Please note that having both device-mapper-multipath and EMC PowerPath on the same system may result in kernel panics. Refer to RedHat's article : https://access.redhat.com/site/solutions/110553

Space Reclamation

You will want to make sure that space reclamation is configured on your Linux Host so that you do not run out of space.  For more information please see this KB: Reclaiming Space on Linux

ActiveCluster

Additional multipath settings are required for ActiveCluster. Please see ActiveCluster Requirements and Best Practices.

SCSI Unit Attentions

The Linux kernel has been enhanced to enable userspace to respond to certain SCSI Unit Attention conditions received from SCSI devices via the udev event mechanism. The FlashArray using version 5.0 and later supports the following SCSI Unit Attentions:

Description ASC ASCQ
CAPACITY DATA HAS CHANGED 0x2A 0x09
ASYMMETRIC ACCESS STATE CHANGED 0x2A 0x06
REPORTED LUNS DATA HAS CHANGED 0x3F 0x0E

With these SCSI Unit Attentions, it is possible to have the Linux initiator auto-rescan on these storage configuration changes. The requirement for auto-rescan support in RHEL/Centos is the libstoragemgmt-udev package. On installing this package a udev rule is installed, 90-scsi-ua.rules. Uncomment the supported Unit Attentions and reload the udev service to pick up the new rules:

[root@host ~]# cat 90-scsi-ua.rules 
#ACTION=="change", SUBSYSTEM=="scsi", ENV{SDEV_UA}=="INQUIRY_DATA_HAS_CHANGED", TEST=="rescan", ATTR{rescan}="x"
ACTION=="change", SUBSYSTEM=="scsi", ENV{SDEV_UA}=="CAPACITY_DATA_HAS_CHANGED", TEST=="rescan", ATTR{rescan}="x"
#ACTION=="change", SUBSYSTEM=="scsi", ENV{SDEV_UA}=="THIN_PROVISIONING_SOFT_THRESHOLD_REACHED", TEST=="rescan", ATTR{rescan}="x"
#ACTION=="change", SUBSYSTEM=="scsi", ENV{SDEV_UA}=="MODE_PARAMETERS_CHANGED", TEST=="rescan", ATTR{rescan}="x"
ACTION=="change", SUBSYSTEM=="scsi", ENV{SDEV_UA}=="REPORTED_LUNS_DATA_HAS_CHANGED", RUN+="scan-scsi-target $env{DEVPATH}"
[root@host ~]# udevadm control --reload-rules && udevadm trigger