Adding Documentation/filesystems/zufs.txt. Adding some Documentation first. So to give the reviewer of the coming patch-set. Some background and overview of the all system. [v2] Incorporated Randy's few comments. Randy Please give it an harder review? CC: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> Signed-off-by: Boaz Harrosh <boazh@xxxxxxxxxx> --- Documentation/filesystems/zufs.txt | 386 +++++++++++++++++++++++++++++ 1 file changed, 386 insertions(+) create mode 100644 Documentation/filesystems/zufs.txt diff --git a/Documentation/filesystems/zufs.txt b/Documentation/filesystems/zufs.txt new file mode 100644 index 000000000000..2a347a446aa7 --- /dev/null +++ b/Documentation/filesystems/zufs.txt @@ -0,0 +1,386 @@ +ZUFS - Zero-copy User-mode FileSystem +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Trees: + git clone https://github.com/NetApp/zufs-zuf -b upstream + git clone https://github.com/NetApp/zufs-zus -b upstream + +patches, comments, questions, requests to: + boazh@xxxxxxxxxx + +Introduction: +~~~~~~~~~~~~~ + +ZUFS - stands for Zero-copy User-mode FS +▪ It is geared towards true zero copy end to end of both data and meta data. +▪ It is geared towards very *low latency*, very high CPU locality, lock-less + parallelism. +▪ Synchronous operations +▪ Numa awareness + + ZUFS is a, from scratch, implementation of a filesystem-in-user-space, which +tries to address the above goals. It is aimed for pmem based FSs. But supports +any other type of FSs + +Glossary and names: +~~~~~~~~~~~~~~~~~~~ + +ZUF - Zero-copy User-mode Feeder + zuf.ko is the Kernel VFS component. Its job is to interface with the Kernel + VFS and dispatch commands to a User-mode application Server. + Uptodate code is found at: + git clone https://github.com/NetApp/zufs-zuf -b upstream + +ZUS - Zero-copy User-mode Server + zufs utilizes a User-mode server application. That takes care of the detailed + communication protocol and correctness with the Kernel. + In turn it utilizes many zusFS Filesystem plugins to implement the actual + on disc Filesystem. + Uptodate code is found at: + git clone https://github.com/NetApp/zufs-zus -b upstream + +zusFS - FS plugins + These are .so loadable modules that implement one or more Filesystem-types + (mount -t xyz). + The zus server communicates with the plugin via a set of function vectors + for the different operations. And establishes communication via defined + structures. + +Filesystem-type: + At startup zus registers with the Kernel one or more Filesystem-type(s) + Associated with the type is a unique type-name (mount -t foofs) + + different info about the fs, like a magic number and so on. + One Server can support many FS-types, in turn each FS-type can mount + multiple super-blocks, each supporting multiple devices. + +Device-Table (MDT) - A zufs FS can support multiple devices + ZUF in Kernel may receive, like any mount command a block-device or none. + For the former if the specified FS-types states so in a special field. + The mount will look for a Device table. A list of devices in a specific + order sitting at some offset on each block-device. The system will then + proceed to open and own all these devices and associate them to the mounting + super-block. + If FS-type specifies a -1 at DT_offset then there is no device table + and a DT of a single device is created. (If we have no devices, none + is specified than we operate without any block devices. (Mount options give + some indication of the storage information)) + The device table has special consideration for pmem devices and will + present the all linear array of devices to zus, as one flat mmap space. + Alternatively all non-pmem devices are also provided an interface + with facility of data movement from pmem to slower devices. + A detailed NUMA info is exported to the Server for maximum utilization. + Each device has an associated NUMA node, so Server can optimize IO to + these devices + +pmem: (Also called t1) + Multiple pmem devices are presented to the server as a single + linear file mmap. Something like /dev/dax. But it is strictly + available only to the specific super-block that owns it. + +Shadow: (For debugging) + "Shadow" is used for debugging the correct persistence of pmem based + filesystems. With pmem if modified a user must call cl_flush/sfence + for the data to be guarantied resistance. This is very hard to test + and time consuming. So for that we invented the shadow. + There is a special mode bit in the MDT header that denotes a shadow + system. In a shadow setup each pmem device is divided in half. First + half is available for FS storage. The second half is a Shadow. IE + each time the FS calls cl_flush or mov_nt the data is then memcopied + to the shadow. + At mount time the Shadow is copied onto the main part. And thous + presenting only those bits that where persisted by the FS. So a simple + remount can simulate a full machine reboot. + The Shadow is presented as the upper part of the mmaped region. IE + the all t1 ranged is repeated again. The zus core code fasilitates + zusFS implementors in accessing this facility + +zufs_dpp_t - Dual port pointer type + At some points in the protocol there are objects that return from zus + (The Server) to the Kernel via a dpp_t. This is a special kind of pointer + It is actually an offset 8 bytes aligned with the 3 low bits specifying + a pool code: [offset = dpp_t & ~0x7] [pool = dpp_t & 0x7] + pool == 0 means the offset is in pmem who's management is by zuf and + a full easy access is provided for zus. + + pool != 0 Is a pre-established file (up to 6 such files per sb) where + the zus has an mmap on the file and the Kernel can access that data + via an offset into the file. + pool == 7 denotes an offset into the application buffers associated + with the current IO. + All dpp_t objects life time rules are strictly defined. + Mainly the primary use of dpp_t is the on-pmem inode structure. Both + zus and zuf can access and change this structure. On any modification + the zus is called so to be notified of any changes, persistence. + More such objects are: Symlinks, xattrs, data-blocks etc... + +Relay-wait-object: + communication between Kernel and server are done via zus-threads that + sleep in Kernel (inside an IOCTL) and wait for commands. Once received + the IOCTL returns operation id executed and the return info is returned via + a new IOCTL call, which then waits for the next operation. + To wake up the sleeping thread we use a Relay-wait-object. Currently + it is two waitqueue_head(s) back to back. + In future we should investigate the use of a new special scheduler object + That switches from thread A to predefined thread ZT context without passing + through the scheduler at all. + (The switching is already very fast, faster then anything currently + in the Kernel. But I believe I can shave another 1 micro off a roundtrip) + +ZT-threads-array: + The novelty of the zufs is the ZT-threads system. 3 threads or more are + pre-created for each active core in the system. + ▪ The thread is AFFINITY set for that single core only. + ▪ Special communication file per ZT (O_TMPFILE + IOCTL_ZUFS_INIT) + At initialization the ZT thread communicates through a ZT_INIT ioctl + and registers as the handler of that core (Channel) + ▪ Also for each ZT, Kernel allocates an IOCTL-buffer that is directly + accessed by Kernel. In turn that IOCTL-buffer is mmaped by zus + for the Server access of that communication buffer. (This is for zero + copy operations as well as avoiding the smem memory barrier) + ▪ IOCTL_ZU_WAIT_OPT – threads sleeps in Kernel waiting for an operation + via the IOCTL_ZU_WAIT_OPT call. + + ▪ On operation dispatch current CPU's ZT free channel is selected. + Operation info is set into the IOCTL-buffer, the ZT is woken and the + application thread is put to sleep. + ▪ After execution, ZT returns to kernel (IOCTL_ZU_WAIT_OPT), app is released, + Server wait for new operation on that CPU. + ▪ Each ZT has a cyclic logic. Each call to IOCTL_ZU_WAIT_OPT from Server + returns the results of the previous operation, before going to sleep + waiting to receive a new operation. + zus zuf-zt application + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + ---> IOCTL_ZU_WAIT_OPT if (app-waiting) + | wake-up-application -> return to app + | FS-WAIT + | | <- POSIX call + | V <- fs-wake-up(dispatch) + | <- return with new command + |--<- do_new_operation + +ZUS-mount-thread: + The system utilizes a single mount thread. (This thread is not affinity to any + core). + ▪ It will first Register all FS-types supported by this Server (By calling + all zusFS plugins to register their supported types). Once done + ▪ As above, the thread sleeps in Kernel via the IOCTL_ZU_MOUNT call. + ▪ When the Kernel receives a mount request (vfs calles the fs_type->mount opt) + a mount is dispatched back to zus. + ▪ NOTE: That only on very first mount the above ZT-threads-array is created + the same ZT-array is then used for all super-blocks in the system + ▪ As part of the mount command in the context of this same mount-thread + a call to IOCTL_ZU_GRAB_PMEM will establish an interface to the pmem + Associated with this super_block + ▪ On return like above a new call to IOCTL_ZU_MOUNT will return info of the + mount before sleeping in kernel waiting for a new dispatch. All SB info + is provided to zuf, including the root inode info. Kernel then proceeds + to complete the mount call. + ▪ NOTE that since there is a single mount thread lots of FS-registration + super_block and pmem management are lockless. + +Philosophy of operations: +~~~~~~~~~~~~~~~~~~~~~~~~~ + +1. [zuf-root] + +On module load (zuf.ko) A special pseudo FS is mounted on /sys/fs/zuf. This is +called zuf-root. +The zuf-root has no visible files. All communication is done via special-files. +special-files are open(O_TMPFILE) and establish a special role via an +IOCTL. (Example above ZT-thread is one such special file) +All communications with the server are done via the zuf-root. Each root owns +many FS-types and each FS-type owns many super-blocks of this type. All Sharing +the same communication channels. +Since all FS-type Servers live in the same zus application address space, at +times. If the administrator wants to separate between different servers, he/she +can mount a new zuf-root and point a new server instance on that new mount, +registering other FS-types on that other instance. The all communication array +will then be duplicated as well. +(Otherwise pointing a new server instance on a busy root will return an error) + +2. [zus server start] + ▪ On load all configured zusFS plugins are loaded. + ▪ The Server starts by starting a single mount thread. + ▪ It than proceeds to register with Kernel all FS-types it will support. + (This is done on the single mount thread, so FS-registration and + mount/umount operate in a single thread and therefor need not any locks) + ▪ Sleeping in the Kernel on a special-file of that zuf-root. waiting for + a mount command. + +3. [mount -t xyz] + [In Kernel] + ▪ If xyz was registered above as part of the Server startup. the regular + mount command will come to the zuf module with a zuf_mount() call. with + the xyz-FS-info. In turn this points to a zuf-root. + ▪ Code than proceed to load a device-table of devices as specified above. + It then establishes an multi_devices object with a specific sb_id. + ▪ It proceeds to call mount_bdev. Always with the same main-device + thous fully sporting automatic bind mounts. Even if different + devices are given to the mount command. + ▪ In zuf_fill_super it will then dispatch (awaken) the mount thread + specifying two parameters. One the FS-type to mount, and then + the sb_id Associated with this super_block. + + [In zus] + ▪ A zus_super_block_info is allocated. + ▪ zus calls PMEM_GRAB(sb_id) to establish a direct mapping to its + pmem devices. On return we have full access to our PMEM + + ▪ ZT-threads-array + If this is the first mount the ZT-threads-array is created and + established. The mount thread will wait until all zt-threads finished + initialization and ready to rock. + ▪ Root-zus_inode is loaded and is returned to kernel + ▪ More info about the mount like block sizes and so on are returned to kernel. + + [In Kernel] + The zuf_fill_super is finalized vectors established and we have a new + super_block ready for operations. + +4. An FS operation like create or WRITE/READ and so on arrives from application + via VFS. Eventually an Operation is dispatched to zus: + ▪ A special per-operation descriptor is filled up with all parameters. + ▪ A current CPU channel is grabbed. the operation descriptor is put on + that channel (ZT). Including get_user_pages or Kernel-pages associated + with this OPT. + ▪ The ZT is awaken, app thread put to sleep. + ▪ Optionally in ZT context pages are mapped to that ZT-vma. This is so we + are sure the map is only on a single core. And no other core's TLB is + affected. + ▪ ZT thread is returned to user-space. + ▪ In ZT context the zus Server calls the appropriate zusFS->operation + vector. Output params filled. + ▪ zus calls again with an IOCTL_ZU_WAIT_OPT with the same descriptor + to return the requested info. + ▪ At Kernel (zuf) the app thread is awaken with the results, and the + ZT thread goes back to sleep waiting a new operation. + + ZT rules: + A ZT thread should try to minimize it's sleeps. it might take locks + In which case we will see that the same CPU channel is reentered via another + application/thread. But now that CPU channel is taken. What we do is we + utilize a few channels (ZTs) per core and those threads may grab another + channel. But this only postpones the problem. On a busy contended system, + all such channels will be consumed. If all channels are taken the + application thread is put on a busy scheduling wait until a channel can + be grabbed. + If The server needs to sleep for a long time it should utilize the + ZUFS_ASYNC return option. The app is then kept sleeping on an + operation-context object and the ZT freed for foreground operation. + At some point in time when the server completes the delayed operation + it will notify the Kernel with a special async IO-context cookie. + And the app will be awakened. + +4. On umount the operation is reversed and all resources are released. +5. In case of an application or Server crash, all resources are Associated + with files, on file_release these resources are caught and freed. + +Objects and life-time +~~~~~~~~~~~~~~~~~~~~~ + +Each Kernel object type has an assosiated zus Server object type who's life +time is governed by the life-time of the Kernel object. Therefor the Server's +job is easy because it need not establish any object caches / hashes and so on. + +Inside zus all objects are allocated by the zusFS plugin. So in turn it can +allocate a bigger space for its own private data and access it via the +container_off() coding pattern. So when I say below a zus-object I mean both +zus public part + zusFS private part of the same object. + +All operations return a User-mode pointer that are opaque to the the Kernel +code, they are just a cookie which is returned back to zus, when needed. +At times when we want the Kernel to have direct access to a zus object like +zufs_inode, along with the cookie we also return a dpp_t, with a defined +structure. + +Kernel object | zus object | Kernel access (via dpp_t) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +zuf_fs_type + file_system_type | zus_fs_info | no + +zuf_sb_info + super_block | zus_sb_info | no + +zuf_inode_info | | + vfs_inode | zus_inode_info | no + zufs_inode * | zufs_inode * | yes + synlink * | char-array | yes + xattr** | zus_xattr | yes + +When a Kernel object's time is to die, a final call to zus is +dispatched so the associated object can also be freed. Which means +that on memory pressure when object caches are evicted also the zus +memory resources are freed. + + +How to use zufs: +~~~~~~~~~~~~~~~~ + +The most updated documentation of how to use the latest code bases +is the script (set of scripts) at fs/do-zu/zudo on the zus git tree + +We the developers at Netapp use this script to mount and test our +latest code. So any new Secret will be found in these scripts. Please +read them as the ultimate source of how to operate things. + +We assume you cloned these git trees: +[]$ mkdir zufs; cd zufs +[]$ git clone https://github.com/NetApp/zufs-zuf -b upstream +[]$ git clone https://github.com/NetApp/zufs-zuf -b upstream + +This will create the following trees +zufs/zus - Source code for Server +zufs/zuf - Linux Kernel source tree to compile and install on your machine + +Also specifically: +zufs/zus/fs/do-zu/zudo - script Documenting how to run things + +[]$ cd zufs + +First time +[] zus/fs/do-zu/zudo +this will create a file: + zus/fs/do-zu/zu.conf + +Edit this file for your environment. Devices, mount-point and so on. +On first run an example file will be created for you. Fill in the +blanks. Most params can stay as is in most cases + +Now lets start running: + +[1]$ zus/fs/do-zu/zudo mkfs +This will run the proper mkfs command selected at zu.conf file +with the proper devices. + +[2]$ zus/fs/do-zu/zudo zuf-insmod +This loads the zuf.ko module + +[3]$ zus/fs/do-zu/zudo zuf-root +This mounts the zuf-root FS above on /sys/fs/zuf (automatically created in [2]) + +[4]$ zus/fs/do-zu/zudo zus-up +This runs the zus daemon in the background + +[5]$ zus/fs/do-zu/zudo mount +This mount the mkfs FS above on the specified dir in zu.conf + +To run all the 5 commands above at once do: +[]$ zus/fs/do-zu/zudo up + +To undo all the above in reverse order do: +[]$ zus/fs/do-zu/zudo down + +And the most magic command is: +[]$ zus/fs/do-zu/zudo again +Will do a "down", then update-mods, then "up" +(update-mods is a special script to copy the latest compiled binaries) + +Now you are ready for some: +[]$ zus/fs/do-zu/zudo xfstest +xfstests is assumed to be installed in the regular /opt/xfstests dir + +Again please see inside the scripts what each command does +these scripts are the ultimate Documentation, do not believe +anything I'm saying here. (Because it is outdated by now) + +Have a nice day -- 2.20.1
