forked from vitalif/vitastor
Correct some typos in README, add note about qemu-img
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README.md
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README.md
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@ -11,7 +11,7 @@ with configurable redundancy (replication or erasure codes/XOR).
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## Features
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Vitastor is currently a pre-release, a lot of features is missing and you can still expect
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Vitastor is currently a pre-release, a lot of features are missing and you can still expect
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breaking changes in the future. However, the following is implemented:
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- Basic part: highly-available block storage with symmetric clustering and no SPOF
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@ -74,14 +74,14 @@ Some basic terms for people not familiar with Ceph:
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Architectural differences from Ceph:
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- Vitastor primary focus is on SSDs. Proper SSD+HDD optimizations may be added in the future, though.
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- Vitastor's primary focus is on SSDs. Proper SSD+HDD optimizations may be added in the future, though.
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- Vitastor OSD is (and will always be) single-threaded. If you want to dedicate more than 1 core
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per drive you should run multiple OSDs each on a different partition of the drive.
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Vitastor isn't CPU-hungry though (as opposed to Ceph), so 1 core is sufficient in a lot of cases.
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- Metadata and journal are always kept in memory. Metadata size depends linearly on drive capacity
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and data store block size which is 128 KB by default. With 128 KB blocks, metadata should occupy
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around 512 MB per 1 TB (which is still less than Ceph wants). Journal doesn't have to be big,
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the example test below was conducted with only 16 MB journal. Big journal is probably even
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the example test below was conducted with only 16 MB journal. A big journal is probably even
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harmful as dirty write metadata also take some memory.
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- Vitastor storage layer doesn't have internal copy-on-write or redirect-write. I know that maybe
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it's possible to create a good copy-on-write storage, but it's much harder and makes performance
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@ -97,12 +97,14 @@ Architectural differences from Ceph:
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- Recovery process is per-object (per-block), not per-PG. Also there are no PGLOGs.
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- Monitors don't store data. Cluster configuration and state is stored in etcd in simple human-readable
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JSON structures. Monitors only watch cluster state and handle data movement.
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Thus Vitastor's Monitor isn't a critical component of the system and is more similar to Ceph's Manager.
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Vitastor's Monitor is implemented in node.js.
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- PG distribution isn't based on consistent hashes. All PG mappings are stored in etcd.
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Rebalancing PGs between OSDs is done by mathematical optimization - data distribution problem
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is reduced to a linear programming problem and solved by lp_solve. This allows for almost
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perfect (96-99% uniformity compared to Ceph's 80-90%) data distribution is most cases, ability
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perfect (96-99% uniformity compared to Ceph's 80-90%) data distribution in most cases, ability
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to map PGs by hand without breaking rebalancing logic, reduced OSD peer-to-peer communication
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(on average, OSDs have less peers) and less data movement. It also probably has a drawback -
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(on average, OSDs have fewer peers) and less data movement. It also probably has a drawback -
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this method may fail in very large clusters, but up to several hundreds of OSDs it's perfectly fine.
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It's also easy to add consistent hashes in the future if something proves their necessity.
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- There's no separate CRUSH layer. You select pool redundancy scheme, placement root, failure domain
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@ -112,7 +114,7 @@ Architectural differences from Ceph:
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The most important thing for fast storage is latency, not parallel iops.
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Best possible latency is achieved with one thread and queue depth of 1 which basically means
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The best possible latency is achieved with one thread and queue depth of 1 which basically means
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"client load as low as possible". In this case IOPS = 1/latency, and this number doesn't
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scale with number of servers, drives, server processes or threads and so on.
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Single-threaded IOPS and latency numbers only depend on *how fast a single daemon is*.
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@ -121,7 +123,7 @@ Why is it important? It's important because some of the applications *can't* use
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queue depth greater than 1 because their task isn't parallelizable. A notable example
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is any ACID DBMS because all of them write their WALs sequentially with fsync()s.
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fsync, by the way, is another important thing often missing in benchmarks. Point is
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fsync, by the way, is another important thing often missing in benchmarks. The point is
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that drives have cache buffers and don't guarantee that your data is actually persisted
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until you call fsync() which is translated to a FLUSH CACHE command by the OS.
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@ -132,9 +134,9 @@ number is around 1000-2000 iops with fsync.
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Server SSDs often have supercapacitors that act as a built-in UPS and allow the drive
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to flush its DRAM cache to the persistent flash storage when a power loss occurs.
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This makes them perform with and without fsync equally well. This feature is called
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This makes them perform equally well with and without fsync. This feature is called
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"Advanced Power Loss Protection" by Intel; other vendors either call it similarly
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or just describe it like "Full Capacitor-Based Power Loss Protection".
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or directly as "Full Capacitor-Based Power Loss Protection".
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All software-defined storages that I currently know are slow in terms of latency.
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Notable examples are Ceph and internal SDSes used by cloud providers like Amazon, Google,
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@ -234,7 +236,8 @@ T8Q64 read test was conducted over 1 larger inode (3.2T) from all hosts (every h
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Vitastor has no performance penalties related to running multiple clients over a single inode.
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If conducted from one node with all primary OSDs moved to other nodes the result was slightly lower (689000 iops),
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this is because all operations resulted in network roundtrips between the client and the primary OSD.
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When fio is colocated with OSDs (like in Ceph benchmarks), 1/4 of the read workload actually uses the loopback network.
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When fio was colocated with OSDs (like in Ceph benchmarks above), 1/4 of the read workload actually
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used the loopback network.
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Vitastor was configured with: `--disable_data_fsync true --immediate_commit all --flusher_count 8
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--disk_alignment 4096 --journal_block_size 4096 --meta_block_size 4096
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@ -248,13 +251,17 @@ Vitastor was configured with: `--disable_data_fsync true --immediate_commit all
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- Install lp_solve.
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- Install etcd.
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- Install node.js 12 or newer.
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- Install gcc and g++ 9.x.
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- Clone https://yourcmc.ru/git/vitalif/vitastor/ with submodules.
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- Install QEMU 4.x or 5.x, get its source, begin to build it, stop the build and copy headers:
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- `<qemu>/include` → `<vitastor>/qemu/include`
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- Debian:
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* Use qemu packages from the main repository
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* `<qemu>/b/qemu/config-host.h` → `<vitastor>/qemu/b/qemu/config-host.h`
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* `<qemu>/b/qemu/qapi` → `<vitastor>/qemu/b/qemu/qapi`
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- CentOS:
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- CentOS 8:
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* Use qemu packages from the Advanced-Virtualization repository. To enable it, run
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`yum install centos-release-advanced-virtualization.noarch` and then `yum install qemu`
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* `<qemu>/config-host.h` → `<vitastor>/qemu/b/qemu/config-host.h`
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* `<qemu>/qapi` → `<vitastor>/qemu/b/qemu/qapi`
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- `config-host.h` and `qapi` are required because they contain generated headers
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@ -295,9 +302,14 @@ and calculate disk offsets almost by hand. This will be fixed in near future.
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which is the number of dirty journal sectors that may be written to at the same time.
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- `systemctl start vitastor.target` everywhere.
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- Start any number of monitors: `cd mon; node mon-main.js --etcd_url 'http://10.115.0.10:2379,http://10.115.0.11:2379,http://10.115.0.12:2379,http://10.115.0.13:2379' --etcd_prefix '/vitastor' --etcd_start_timeout 5`.
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- At this point, one the monitors will configure PGs and OSDs will start them.
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- At this point, one of the monitors will configure PGs and OSDs will start them.
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- You can check PG states with `etcdctl get --prefix /vitastor/pg/state`. All PGs should become 'active'.
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- Run tests with (for example): `fio -thread -ioengine=./libfio_cluster.so -name=test -bs=4M -direct=1 -iodepth=16 -rw=write -etcd=10.115.0.10:2379/v3 -pool=1 -inode=1 -size=400G`.
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- Upload VM disk image with qemu-img (for example):
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```
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LD_PRELOAD=./qemu_driver.so qemu-img convert -f qcow2 debian10.qcow2 -p
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-O raw 'vitastor:etcd_host=10.115.0.10\:2379/v3:pool=1:inode=1:size=2147483648'
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```
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- Run QEMU with (for example):
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```
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LD_PRELOAD=./qemu_driver.so qemu-system-x86_64 -enable-kvm -m 1024
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@ -313,7 +325,7 @@ and calculate disk offsets almost by hand. This will be fixed in near future.
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and OSDs don't check if it fills up.
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- Object deletion requests may currently lead to unfound objects on crashes because
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proper handling of deletions in a cluster requires a "three-phase cleanup process"
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and it's not currently implemented. In fact, even though deletion requests are
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and it's currently not implemented. In fact, even though deletion requests are
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implemented, there's no user tool to delete anything from the cluster yet :).
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Of course I'll create such tool, but its first implementation will be vulnerable to this issue.
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It's not a big deal though, because you'll be able to just repeat the deletion request
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@ -328,6 +340,9 @@ and calculate disk offsets almost by hand. This will be fixed in near future.
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- I don't care about C++ "best practices" like RAII or proper inheritance or usage of
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smart pointers or whatever and I don't intend to change my mind, so if you're here
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looking for ideal reference C++ code, this probably isn't the right place.
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- I like node.js better than any other dynamically-typed language interpreter
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because it's faster than any other interpreter in the world, has neutral C-like
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syntax and built-in event loop. That's why Monitor is implemented in node.js.
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## Author and License
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