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118 lines
4.1 KiB
Plaintext
118 lines
4.1 KiB
Plaintext
ClickHouse administration tips.
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CPU
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SSE 4.2 instruction set support is required. Most recent (since 2008) CPUs have this instruction set.
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When choosing between CPU with more cores and slightly less frequency and CPU with less cores and more frequency, choose first.
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For example, 16 cores with 2600 MHz is better than 8 cores with 3600 MHz.
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Hyper-Threading
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Don't disable hyper-threading. Some queries will benefit from hyper-threading and some will not.
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Turbo-Boost
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Don't disable turbo-boost. It will do significant performance gain on typical load.
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Use 'turbostat' tool to show real CPU frequency under load.
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CPU scaling governor
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Always use 'performance' scaling governor. 'ondemand' scaling governor performs much worse even on constantly high demand.
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sudo echo 'performance' | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
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CPU throttling
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Your CPU could be overheated. Use 'dmesg' to show if it was thermal throttled.
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Your CPU could be power capped in datacenter. Use 'turbostat' tool under load to monitor that.
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RAM
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For small amount of data (up to ~200 GB compressed) prefer to use as much RAM as data volume.
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For larger amount of data, if you run interactive (online) queries, use reasonable amount of RAM (128 GB or more) to hot data fit in page cache.
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Even for data volumes of ~50 TB per server, using 128 GB of RAM is much better for query performance than 64 GB.
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Swap
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Disable swap. The only possible reason to not disable swap is when you are running ClickHouse on your personal laptop/desktop.
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Huge pages
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Disable transparent huge pages. It interfers badly with memory allocators, leading to major performance degradation.
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echo 'never' | sudo tee /sys/kernel/mm/transparent_hugepage/enabled
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Use 'perf top' to monitor time spent in kernel on doing memory management.
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Don't allocate permanent huge pages.
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Storage subsystem
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If you could afford SSD, use SSD.
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Otherwise use HDD. SATA HDDs 7200 RPM are Ok.
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Prefer more servers with inplace storage to less servers with huge disk shelves.
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Of course you could use huge disk shelves for archive storage with rare queries.
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RAID
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When using HDDs, you could use RAID-10, RAID-5, RAID-6 or RAID-50.
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Use Linux software RAID (mdadm). Better to not use LVM.
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When creating RAID-10, choose 'far' layout.
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Prefer RAID-10 if you could afford it.
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Don't use RAID-5 on more than 4 HDDs - use RAID-6 or RAID-50. RAID-6 is better.
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When using RAID-5, RAID-6, or RAID-50, always increase stripe_cache_size, because default setting is awful.
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echo 4096 | sudo tee /sys/block/md2/md/stripe_cache_size
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Exact number is calculated from number of devices and chunk size: 2 * num_devices * chunk_size_in_bytes / 4096.
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Chunk size 1024K is Ok for all RAID configurations.
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Never use too small or too large chunk sizes.
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On SSDs, you could use RAID-0.
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Regardless to RAID, always use replication for data safety.
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Enable NCQ. Use high queue depth. Use CFQ scheduler for HDDs and noop for SSDs. Don't lower readahead setting.
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Enable write cache on HDDs.
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Filesystem
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Ext4 is Ok. Mount with noatime,nobarrier.
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XFS is Ok too, but less tested with ClickHouse.
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Most other filesystems should work fine. Filesystems with delayed allocation are better.
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Linux kernel
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Don't use too old Linux kernel. For example, on 2015, 3.18.19 is Ok.
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You could use Yandex kernel: https://github.com/yandex/smart
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- it gives at least 5% performance increase.
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Network
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When using IPv6, you must increase route cache.
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Linux kernels before 3.2 has awful bugs in IPv6 implementation.
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Prefer at least 10 Gbit network. 1 Gbit will also work, but much worse for repairing replicas with tens of terabytes of data and for processing huge distributed queries with much intermediate data.
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ZooKeeper
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Probably you already have ZooKeeper for another purposes.
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It's Ok to use existing ZooKeeper installation if it is not overloaded.
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Use recent version of ZooKeeper. At least 3.5 is Ok. Version in your linux package repository might be outdated.
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With default settings, ZooKeeper have time bomb:
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"A ZooKeeper server will not remove old snapshots and log files when using the default configuration (see autopurge below), this is the responsibility of the operator."
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You need to defuse the bomb.
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