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alistair23-linux/Documentation/block/queue-sysfs.rst

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=================
Queue sysfs files
=================
This text file will detail the queue files that are located in the sysfs tree
for each block device. Note that stacked devices typically do not export
any settings, since their queue merely functions are a remapping target.
These files are the ones found in the /sys/block/xxx/queue/ directory.
Files denoted with a RO postfix are readonly and the RW postfix means
read-write.
add_random (RW)
---------------
This file allows to turn off the disk entropy contribution. Default
value of this file is '1'(on).
chunk_sectors (RO)
------------------
This has different meaning depending on the type of the block device.
For a RAID device (dm-raid), chunk_sectors indicates the size in 512B sectors
of the RAID volume stripe segment. For a zoned block device, either host-aware
or host-managed, chunk_sectors indicates the size in 512B sectors of the zones
of the device, with the eventual exception of the last zone of the device which
may be smaller.
dax (RO)
--------
This file indicates whether the device supports Direct Access (DAX),
used by CPU-addressable storage to bypass the pagecache. It shows '1'
if true, '0' if not.
discard_granularity (RO)
------------------------
This shows the size of internal allocation of the device in bytes, if
reported by the device. A value of '0' means device does not support
the discard functionality.
discard_max_hw_bytes (RO)
-------------------------
Devices that support discard functionality may have internal limits on
the number of bytes that can be trimmed or unmapped in a single operation.
The discard_max_bytes parameter is set by the device driver to the maximum
number of bytes that can be discarded in a single operation. Discard
requests issued to the device must not exceed this limit. A discard_max_bytes
value of 0 means that the device does not support discard functionality.
discard_max_bytes (RW)
----------------------
While discard_max_hw_bytes is the hardware limit for the device, this
setting is the software limit. Some devices exhibit large latencies when
large discards are issued, setting this value lower will make Linux issue
smaller discards and potentially help reduce latencies induced by large
discard operations.
discard_zeroes_data (RO)
------------------------
Obsolete. Always zero.
fua (RO)
--------
Whether or not the block driver supports the FUA flag for write requests.
FUA stands for Force Unit Access. If the FUA flag is set that means that
write requests must bypass the volatile cache of the storage device.
hw_sector_size (RO)
-------------------
This is the hardware sector size of the device, in bytes.
io_poll (RW)
------------
When read, this file shows whether polling is enabled (1) or disabled
(0). Writing '0' to this file will disable polling for this device.
Writing any non-zero value will enable this feature.
io_poll_delay (RW)
------------------
If polling is enabled, this controls what kind of polling will be
performed. It defaults to -1, which is classic polling. In this mode,
the CPU will repeatedly ask for completions without giving up any time.
If set to 0, a hybrid polling mode is used, where the kernel will attempt
to make an educated guess at when the IO will complete. Based on this
guess, the kernel will put the process issuing IO to sleep for an amount
of time, before entering a classic poll loop. This mode might be a
little slower than pure classic polling, but it will be more efficient.
If set to a value larger than 0, the kernel will put the process issuing
IO to sleep for this amount of microseconds before entering classic
polling.
io_timeout (RW)
---------------
io_timeout is the request timeout in milliseconds. If a request does not
complete in this time then the block driver timeout handler is invoked.
That timeout handler can decide to retry the request, to fail it or to start
a device recovery strategy.
iostats (RW)
-------------
This file is used to control (on/off) the iostats accounting of the
disk.
logical_block_size (RO)
-----------------------
This is the logical block size of the device, in bytes.
max_discard_segments (RO)
-------------------------
The maximum number of DMA scatter/gather entries in a discard request.
max_hw_sectors_kb (RO)
----------------------
This is the maximum number of kilobytes supported in a single data transfer.
max_integrity_segments (RO)
---------------------------
Maximum number of elements in a DMA scatter/gather list with integrity
data that will be submitted by the block layer core to the associated
block driver.
max_sectors_kb (RW)
-------------------
This is the maximum number of kilobytes that the block layer will allow
for a filesystem request. Must be smaller than or equal to the maximum
size allowed by the hardware.
max_segments (RO)
-----------------
Maximum number of elements in a DMA scatter/gather list that is submitted
to the associated block driver.
max_segment_size (RO)
---------------------
Maximum size in bytes of a single element in a DMA scatter/gather list.
minimum_io_size (RO)
--------------------
This is the smallest preferred IO size reported by the device.
nomerges (RW)
-------------
This enables the user to disable the lookup logic involved with IO
merging requests in the block layer. By default (0) all merges are
enabled. When set to 1 only simple one-hit merges will be tried. When
set to 2 no merge algorithms will be tried (including one-hit or more
complex tree/hash lookups).
nr_requests (RW)
----------------
This controls how many requests may be allocated in the block layer for
read or write requests. Note that the total allocated number may be twice
this amount, since it applies only to reads or writes (not the accumulated
sum).
blkcg: implement per-blkg request allocation Currently, request_queue has one request_list to allocate requests from regardless of blkcg of the IO being issued. When the unified request pool is used up, cfq proportional IO limits become meaningless - whoever grabs the next request being freed wins the race regardless of the configured weights. This can be easily demonstrated by creating a blkio cgroup w/ very low weight, put a program which can issue a lot of random direct IOs there and running a sequential IO from a different cgroup. As soon as the request pool is used up, the sequential IO bandwidth crashes. This patch implements per-blkg request_list. Each blkg has its own request_list and any IO allocates its request from the matching blkg making blkcgs completely isolated in terms of request allocation. * Root blkcg uses the request_list embedded in each request_queue, which was renamed to @q->root_rl from @q->rq. While making blkcg rl handling a bit harier, this enables avoiding most overhead for root blkcg. * Queue fullness is properly per request_list but bdi isn't blkcg aware yet, so congestion state currently just follows the root blkcg. As writeback isn't aware of blkcg yet, this works okay for async congestion but readahead may get the wrong signals. It's better than blkcg completely collapsing with shared request_list but needs to be improved with future changes. * After this change, each block cgroup gets a full request pool making resource consumption of each cgroup higher. This makes allowing non-root users to create cgroups less desirable; however, note that allowing non-root users to directly manage cgroups is already severely broken regardless of this patch - each block cgroup consumes kernel memory and skews IO weight (IO weights are not hierarchical). v2: queue-sysfs.txt updated and patch description udpated as suggested by Vivek. v3: blk_get_rl() wasn't checking error return from blkg_lookup_create() and may cause oops on lookup failure. Fix it by falling back to root_rl on blkg lookup failures. This problem was spotted by Rakesh Iyer <rni@google.com>. v4: Updated to accomodate 458f27a982 "block: Avoid missed wakeup in request waitqueue". blk_drain_queue() now wakes up waiters on all blkg->rl on the target queue. Signed-off-by: Tejun Heo <tj@kernel.org> Acked-by: Vivek Goyal <vgoyal@redhat.com> Cc: Wu Fengguang <fengguang.wu@intel.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2012-06-26 16:05:44 -06:00
To avoid priority inversion through request starvation, a request
queue maintains a separate request pool per each cgroup when
CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such
per-block-cgroup request pool. IOW, if there are N block cgroups,
each request queue may have up to N request pools, each independently
blkcg: implement per-blkg request allocation Currently, request_queue has one request_list to allocate requests from regardless of blkcg of the IO being issued. When the unified request pool is used up, cfq proportional IO limits become meaningless - whoever grabs the next request being freed wins the race regardless of the configured weights. This can be easily demonstrated by creating a blkio cgroup w/ very low weight, put a program which can issue a lot of random direct IOs there and running a sequential IO from a different cgroup. As soon as the request pool is used up, the sequential IO bandwidth crashes. This patch implements per-blkg request_list. Each blkg has its own request_list and any IO allocates its request from the matching blkg making blkcgs completely isolated in terms of request allocation. * Root blkcg uses the request_list embedded in each request_queue, which was renamed to @q->root_rl from @q->rq. While making blkcg rl handling a bit harier, this enables avoiding most overhead for root blkcg. * Queue fullness is properly per request_list but bdi isn't blkcg aware yet, so congestion state currently just follows the root blkcg. As writeback isn't aware of blkcg yet, this works okay for async congestion but readahead may get the wrong signals. It's better than blkcg completely collapsing with shared request_list but needs to be improved with future changes. * After this change, each block cgroup gets a full request pool making resource consumption of each cgroup higher. This makes allowing non-root users to create cgroups less desirable; however, note that allowing non-root users to directly manage cgroups is already severely broken regardless of this patch - each block cgroup consumes kernel memory and skews IO weight (IO weights are not hierarchical). v2: queue-sysfs.txt updated and patch description udpated as suggested by Vivek. v3: blk_get_rl() wasn't checking error return from blkg_lookup_create() and may cause oops on lookup failure. Fix it by falling back to root_rl on blkg lookup failures. This problem was spotted by Rakesh Iyer <rni@google.com>. v4: Updated to accomodate 458f27a982 "block: Avoid missed wakeup in request waitqueue". blk_drain_queue() now wakes up waiters on all blkg->rl on the target queue. Signed-off-by: Tejun Heo <tj@kernel.org> Acked-by: Vivek Goyal <vgoyal@redhat.com> Cc: Wu Fengguang <fengguang.wu@intel.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2012-06-26 16:05:44 -06:00
regulated by nr_requests.
nr_zones (RO)
-------------
For zoned block devices (zoned attribute indicating "host-managed" or
"host-aware"), this indicates the total number of zones of the device.
This is always 0 for regular block devices.
optimal_io_size (RO)
--------------------
This is the optimal IO size reported by the device.
physical_block_size (RO)
------------------------
This is the physical block size of device, in bytes.
read_ahead_kb (RW)
------------------
Maximum number of kilobytes to read-ahead for filesystems on this block
device.
rotational (RW)
---------------
This file is used to stat if the device is of rotational type or
non-rotational type.
rq_affinity (RW)
----------------
If this option is '1', the block layer will migrate request completions to the
cpu "group" that originally submitted the request. For some workloads this
provides a significant reduction in CPU cycles due to caching effects.
For storage configurations that need to maximize distribution of completion
processing setting this option to '2' forces the completion to run on the
requesting cpu (bypassing the "group" aggregation logic).
scheduler (RW)
--------------
When read, this file will display the current and available IO schedulers
for this block device. The currently active IO scheduler will be enclosed
in [] brackets. Writing an IO scheduler name to this file will switch
control of this block device to that new IO scheduler. Note that writing
an IO scheduler name to this file will attempt to load that IO scheduler
module, if it isn't already present in the system.
write_cache (RW)
----------------
When read, this file will display whether the device has write back
caching enabled or not. It will return "write back" for the former
case, and "write through" for the latter. Writing to this file can
change the kernels view of the device, but it doesn't alter the
device state. This means that it might not be safe to toggle the
setting from "write back" to "write through", since that will also
eliminate cache flushes issued by the kernel.
write_same_max_bytes (RO)
-------------------------
This is the number of bytes the device can write in a single write-same
command. A value of '0' means write-same is not supported by this
device.
wbt_lat_usec (RW)
-----------------
block: hook up writeback throttling Enable throttling of buffered writeback to make it a lot more smooth, and has way less impact on other system activity. Background writeback should be, by definition, background activity. The fact that we flush huge bundles of it at the time means that it potentially has heavy impacts on foreground workloads, which isn't ideal. We can't easily limit the sizes of writes that we do, since that would impact file system layout in the presence of delayed allocation. So just throttle back buffered writeback, unless someone is waiting for it. The algorithm for when to throttle takes its inspiration in the CoDel networking scheduling algorithm. Like CoDel, blk-wb monitors the minimum latencies of requests over a window of time. In that window of time, if the minimum latency of any request exceeds a given target, then a scale count is incremented and the queue depth is shrunk. The next monitoring window is shrunk accordingly. Unlike CoDel, if we hit a window that exhibits good behavior, then we simply increment the scale count and re-calculate the limits for that scale value. This prevents us from oscillating between a close-to-ideal value and max all the time, instead remaining in the windows where we get good behavior. Unlike CoDel, blk-wb allows the scale count to to negative. This happens if we primarily have writes going on. Unlike positive scale counts, this doesn't change the size of the monitoring window. When the heavy writers finish, blk-bw quickly snaps back to it's stable state of a zero scale count. The patch registers a sysfs entry, 'wb_lat_usec'. This sets the latency target to me met. It defaults to 2 msec for non-rotational storage, and 75 msec for rotational storage. Setting this value to '0' disables blk-wb. Generally, a user would not have to touch this setting. We don't enable WBT on devices that are managed with CFQ, and have a non-root block cgroup attached. If we have a proportional share setup on this particular disk, then the wbt throttling will interfere with that. We don't have a strong need for wbt for that case, since we will rely on CFQ doing that for us. Signed-off-by: Jens Axboe <axboe@fb.com>
2016-11-09 12:38:14 -07:00
If the device is registered for writeback throttling, then this file shows
the target minimum read latency. If this latency is exceeded in a given
window of time (see wb_window_usec), then the writeback throttling will start
scaling back writes. Writing a value of '0' to this file disables the
feature. Writing a value of '-1' to this file resets the value to the
default setting.
block: hook up writeback throttling Enable throttling of buffered writeback to make it a lot more smooth, and has way less impact on other system activity. Background writeback should be, by definition, background activity. The fact that we flush huge bundles of it at the time means that it potentially has heavy impacts on foreground workloads, which isn't ideal. We can't easily limit the sizes of writes that we do, since that would impact file system layout in the presence of delayed allocation. So just throttle back buffered writeback, unless someone is waiting for it. The algorithm for when to throttle takes its inspiration in the CoDel networking scheduling algorithm. Like CoDel, blk-wb monitors the minimum latencies of requests over a window of time. In that window of time, if the minimum latency of any request exceeds a given target, then a scale count is incremented and the queue depth is shrunk. The next monitoring window is shrunk accordingly. Unlike CoDel, if we hit a window that exhibits good behavior, then we simply increment the scale count and re-calculate the limits for that scale value. This prevents us from oscillating between a close-to-ideal value and max all the time, instead remaining in the windows where we get good behavior. Unlike CoDel, blk-wb allows the scale count to to negative. This happens if we primarily have writes going on. Unlike positive scale counts, this doesn't change the size of the monitoring window. When the heavy writers finish, blk-bw quickly snaps back to it's stable state of a zero scale count. The patch registers a sysfs entry, 'wb_lat_usec'. This sets the latency target to me met. It defaults to 2 msec for non-rotational storage, and 75 msec for rotational storage. Setting this value to '0' disables blk-wb. Generally, a user would not have to touch this setting. We don't enable WBT on devices that are managed with CFQ, and have a non-root block cgroup attached. If we have a proportional share setup on this particular disk, then the wbt throttling will interfere with that. We don't have a strong need for wbt for that case, since we will rely on CFQ doing that for us. Signed-off-by: Jens Axboe <axboe@fb.com>
2016-11-09 12:38:14 -07:00
throttle_sample_time (RW)
-------------------------
This is the time window that blk-throttle samples data, in millisecond.
blk-throttle makes decision based on the samplings. Lower time means cgroups
have more smooth throughput, but higher CPU overhead. This exists only when
CONFIG_BLK_DEV_THROTTLING_LOW is enabled.
write_zeroes_max_bytes (RO)
---------------------------
For block drivers that support REQ_OP_WRITE_ZEROES, the maximum number of
bytes that can be zeroed at once. The value 0 means that REQ_OP_WRITE_ZEROES
is not supported.
zoned (RO)
----------
This indicates if the device is a zoned block device and the zone model of the
device if it is indeed zoned. The possible values indicated by zoned are
"none" for regular block devices and "host-aware" or "host-managed" for zoned
block devices. The characteristics of host-aware and host-managed zoned block
devices are described in the ZBC (Zoned Block Commands) and ZAC
(Zoned Device ATA Command Set) standards. These standards also define the
"drive-managed" zone model. However, since drive-managed zoned block devices
do not support zone commands, they will be treated as regular block devices
and zoned will report "none".
Jens Axboe <jens.axboe@oracle.com>, February 2009