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Denser containers with PFCacheDenser containers with PFCache
Pavel Emelyanov
ContainerCon, Seattle, 2015
AgendaAgenda
• How to store container files
• Why shared template matters
• What can be deduplicated and what should be
• PFCache
• Q&A
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How to store container filesHow to store container files
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Filesystem
Containerprocesses
How to store container filesHow to store container files
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Filesystem
Containerprocesses
Block device
NetworkHostFilesystem
Hostblock device
Hardware
How to store container files (1)How to store container files (1)
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Filesystem
Containerprocesses
Block device
NetworkHostFilesystem
Hostblock device
Hardware
Chroot()Union FS
How to store container files (2)How to store container files (2)
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Filesystem
Containerprocesses
Block device
NetworkHostFilesystem
Hostblock device
Hardware
Loop deviceZFS ZVolBTRFS subvolumePLoop
What's PLoopWhat's PLoop
• Loop device plus
– AIO for better performance
– Snapshots
– QCOW2-like format for thin provisioning
– Thin provisionong itself
• Upstreaming work in progress
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How to store container files (3)How to store container files (3)
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Filesystem
Containerprocesses
Block device
NetworkHostFilesystem
Hostblock device
Hardware
LVMDM-thin
How to store container files (4)How to store container files (4)
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Filesystem
Containerprocesses
Block device
NetworkHostFilesystem
Hostblock device
Hardware
NBDCeph RBDiSCSI
How to store container files (5)How to store container files (5)
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Filesystem
Containerprocesses
Block device
NetworkHostFilesystem
Hostblock device
Hardware
NFSGFS2OCFSCeph
Containers vs TemplatesContainers vs Templates
• Containers ...
– are massively cloned from pre-created “templates”
– do not have direct access to the underlying (block) storage
• Identical data can be effectively deduplicated
– Higher density
– Lower IO and/or memory consumption
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Who can do shared templatesWho can do shared templates
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Storage OpenVZ Docker LXC
Union FSs + + +
Btrfs +
DM-thin +
PLoop +
Ceph
ZFS +
What can be de-duplicatedWhat can be de-duplicated
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Filesystem
Containerprocesses
Block device Network
What can be de-duplicatedWhat can be de-duplicated
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Filesystem
Containerprocesses
Block device Network
Page cacheCached pages
What can be de-duplicatedWhat can be de-duplicated
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Filesystem
Containerprocesses
Block device Network
Page cacheCached pages
IO flow
What is deduplicatedWhat is deduplicated
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Storage Memory IO
Union FSs + +
Btrfs +/-
DM-thin
PLoop + +
Ceph
ZFS
Additional OpenVZ constraintsAdditional OpenVZ constraints
• Containers disks are independent image files
– Can be easily copied across nodes
– No single (shared) point of failure
• Deduplicated data is volatile
– “Templates” can be lost (e.g. while migrating)
– Too big pool with shared data can be easily shrunk
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Virtuozzo IO stackVirtuozzo IO stack
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Ext4
Containerprocesses
PLoop device
Image file
PF-CachePF-Cache
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Ext4
PLoop device
Cache area
Cache link (xattr)Ext4
Containerprocesses
PLoop device
Image file Image file
Cache and cache link behaviorCache and cache link behavior
• Cache area
– target file name is sha1 sum of the contents
– files are created by user-space daemon
– cache size is limited by ploop
• Cache link
– created automatically upon file creation
– dropped when file is opened for writing
– Is kept during metadata update (chown/chmod)
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Density resultsDensity results
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Future workFuture work
• PLoop is available in OpenVZ & Virtuozzo
– Upstream WIP
• IO deduplication in the upstream
– Issue raied at 2013'th LSFMM
– DM-thin/btrfs IO dedup for containers
– KSM++ for VM-s
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Thank you
Thank you
