I/O scheduling controls how input/output operations will be submitted to
storage. SUSE Linux Enterprise Server offers various I/O algorithms—called
elevators— suiting different workloads. Elevators
can help to reduce seek operations, can prioritize I/O requests, or make
sure, and I/O request is carried out before a given deadline.
Choosing the best suited I/O elevator not only depends on the workload, but on the hardware, too. Single ATA disk systems, SSDs, RAID arrays, or network storage systems, for example, each require different tuning strategies.
SUSE Linux Enterprise Server lets you set a default I/O scheduler at boot-time, which can be changed on the fly per block device. This makes it possible to set different algorithms for e.g. the device hosting the system partition and the device hosting a database.
By default the
Fair Queuing) scheduler is used. Change this default by entering the boot
SCHEDULER is one of
Section 13.2, “Available I/O Elevators” for details.
To change the elevator for a specific device in the running system, run the following command:
SCHEDULER is one of
DEVICE the block device
sda for example).
|Default Schedulter on IBM System z|
On IBM System z the default I/O scheduler for a storage device is set by the device driver.
In the following elevators available on SUSE Linux Enterprise Server are listed. Each elevator has a set of tunable parameters, which can be set with the following command:
VALUE is the desired value for the
the block device.
To find out which elevator is the current default, run the following command. The currently selected scheduler is listed in brackets:
jupiter:~ # cat /sys/block/sda/queue/scheduler noop deadline [cfq]
CFQ is a fairness-oriented
scheduler and is used by default on SUSE Linux Enterprise Server. The algorithm assigns
each thread a time slice in which it is allowed to submit I/O to disk.
This way each thread gets a fair share of I/O throughput. It also allows
assigning tasks I/O priorities which are taken into account during
scheduling decisions (see man 1 ionice). The
CFQ scheduler has the
following tunable parameters:
When a task has no more I/O to submit in its time slice, the I/O
scheduler waits for a while before scheduling the next thread to
improve locality of I/O. For media where locality does not play a big
role (SSDs, SANs with lots of disks) setting
0 can improve the throughput considerably.
This option limits the maximum number of requests that are being
processed by the device at once. The default value is
4. For a storage with several disks, this setting
can unnecessarily limit parallel processing of requests. Therefore,
increasing the value can improve performance although this can cause
that the latency of some I/O may be increased due to more requests
being buffered inside the storage. When changing this value, you can
also consider tuning
(the default value is
2) which limits the maximum
number of asynchronous requests—usually writing
requests—that are submitted in one time slice.
For workloads where the latency of I/O is crucial, setting
1 can help.
A trivial scheduler that just passes down the I/O that comes to it. Useful for checking whether complex I/O scheduling decisions of other schedulers are not causing I/O performance regressions.
In some cases it can be helpful for devices that do I/O scheduling
themselves, as intelligent storage, or devices that do not depend on
mechanical movement, like SSDs. Usually, the
DEADLINE I/O scheduler is
a better choice for these devices, but due to less overhead
NOOP may produce better
performance on certain workloads.
DEADLINE is a latency-oriented
I/O scheduler. Each I/O request has got a deadline assigned. Usually,
requests are stored in queues (read and write) sorted by sector numbers.
maintains two additional queues (read and write) where the requests are
sorted by deadline. As long as no request has timed out, the
“sector” queue is used. If timeouts occur, requests from
the “deadline” queue are served until there are no more
expired requests. Generally, the algorithm prefers reads over writes.
This scheduler can provide a superior throughput over the
CFQ I/O scheduler in
cases where several threads read and write and fairness is not an issue.
For example, for several parallel readers from a SAN and for databases
(especially when using “TCQ” disks). The
DEADLINE scheduler has
the following tunable parameters:
Controls how many reads can be sent to disk before it is possible to
send writes. A value of
3 means, that three read
operations are carried out for one write operation.
Sets the deadline (current time plus the read_expire value) for read operations in milliseconds. The default is 500.
Sets the deadline (current time plus the read_expire value) for read
operations in milliseconds. The default is 500.
Most file systems (XFS, ext3, ext4, reiserfs) send write barriers to disk after fsync or during transaction commits. Write barriers enforce proper ordering of writes, making volatile disk write caches safe to use (at some performance penalty). If your disks are battery-backed in one way or another, disabling barriers may safely improve performance.
Sending write barriers can be disabled using the
barrier=0 mount option (for ext3, ext4, and reiserfs),
or using the
nobarrier mount option (for XFS).
Disabling barriers when disks cannot guarantee caches are properly written in case of power failure can lead to severe file system corruption and data loss.