Filesystems

Beyond the File System

Designing Large Scale File Storage and Serving

Cal Henderson

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Hello!

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Big file systems?

• Too vague!

• What is a file system?

• What constitutes big?

• Some requirements would be nice

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ScalableLooking at storage and serving infrastructures1

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ReliableLooking at redundancy, failure rates, on the fly changes2

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CheapLooking at upfront costs, TCO and lifetimes3

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Four buckets

Storage

Serving

BCP

Cost

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Storage

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The storage stack

File system

Block protocol

RAID

Hardware

ext, reiserFS, NTFS

SCSI, SATA, FC

Mirrors, Stripes

Disks and stuff

File protocol NFS, CIFS, SMB

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Hardware overview

The storage scale

Internal DAS SAN NAS

Lower Higher

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Internal storage

• A disk in a computer– SCSI, IDE, SATA

• 4 disks in 1U is common

• 8 for half depth boxes

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DAS

Direct attached storage

Disk shelf, connected by SCSI/SATA

HP MSA30 – 14 disks in 3U

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SAN

• Storage Area Network

• Dumb disk shelves

• Clients connect via a ‘fabric’

• Fibre Channel, iSCSI, Infiniband– Low level protocols

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NAS

• Network Attached Storage

• Intelligent disk shelf

• Clients connect via a network

• NFS, SMB, CIFS– High level protocols

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Of course, it’s more confusing than that

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Meet the LUN

• Logical Unit Number

• A slice of storage space

• Originally for addressing a single drive:– c1t2d3– Controller, Target, Disk (Slice)

• Now means a virtual partition/volume– LVM, Logical Volume Management

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NAS vs SAN

With SAN, a single host (initiator) owns a single LUN/volume

With NAS, multiple hosts own a single LUN/volume

NAS head – NAS access to a SAN

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SAN Advantages

Virtualization within a SAN offers some nice features:

• Real-time LUN replication

• Transparent backup

• SAN booting for host replacement

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Some Practical Examples

• There are a lot of vendors

• Configurations vary

• Prices vary wildly

• Let’s look at a couple– Ones I happen to have experience with– Not an endorsement ;)

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NetApp Filers

Heads and shelves, up to 500TB in 260U

FC SAN with 1 or 2 NAS heads

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Isilon IQ

• 2U Nodes, 3-96 nodes/cluster, 6-600 TB

• FC/InfiniBand SAN with NAS head on each node

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Scaling

Vertical vs Horizontal

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Vertical scaling

• Get a bigger box

• Bigger disk(s)

• More disks

• Limited by current tech – size of each disk and total number in appliance

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Horizontal scaling

• Buy more boxes

• Add more servers/appliances

• Scales forever*

*sort of

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Storage scaling approaches

• Four common models:

• Huge FS

• Physical nodes

• Virtual nodes

• Chunked space

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Huge FS

• Create one giant volume with growing space– Sun’s ZFS– Isilon IQ

• Expandable on-the-fly?

• Upper limits– Always limited somewhere

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Huge FS

• Pluses– Simple from the application side– Logically simple– Low administrative overhead

• Minuses– All your eggs in one basket– Hard to expand– Has an upper limit

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Physical nodes

• Application handles distribution to multiple physical nodes– Disks, Boxes, Appliances, whatever

• One ‘volume’ per node

• Each node acts by itself

• Expandable on-the-fly – add more nodes

• Scales forever

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Physical Nodes

• Pluses– Limitless expansion– Easy to expand– Unlikely to all fail at once

• Minuses– Many ‘mounts’ to manage– More administration

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Virtual nodes

• Application handles distribution to multiple virtual volumes, contained on multiple physical nodes

• Multiple volumes per node

• Flexible

• Expandable on-the-fly – add more nodes

• Scales forever

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Virtual Nodes

• Pluses– Limitless expansion– Easy to expand– Unlikely to all fail at once– Addressing is logical, not physical– Flexible volume sizing, consolidation

• Minuses– Many ‘mounts’ to manage– More administration

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Chunked space

• Storage layer writes parts of files to different physical nodes

• A higher-level RAID striping

• High performance for large files– read multiple parts simultaneously

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Chunked space

• Pluses– High performance– Limitless size

• Minuses– Conceptually complex– Can be hard to expand on the fly– Can’t manually poke it

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Real Life

Case Studies

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GFS – Google File System

• Developed by … Google

• Proprietary

• Everything we know about it is based on talks they’ve given

• Designed to store huge files for fast access

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GFS – Google File System

• Single ‘Master’ node holds metadata– SPF – Shadow master allows warm swap

• Grid of ‘chunkservers’– 64bit filenames– 64 MB file chunks

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GFS – Google File System

1(a) 2(a)

1(b)

Master

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GFS – Google File System

• Client reads metadata from master then file parts from multiple chunkservers

• Designed for big files (>100MB)

• Master server allocates access leases

• Replication is automatic and self repairing– Synchronously for atomicity

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GFS – Google File System

• Reading is fast (parallelizable)– But requires a lease

• Master server is required for all reads and writes

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MogileFS – OMG Files

• Developed by Danga / SixApart

• Open source

• Designed for scalable web app storage

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MogileFS – OMG Files

• Single metadata store (MySQL)– MySQL Cluster avoids SPF

• Multiple ‘tracker’ nodes locate files

• Multiple ‘storage’ nodes store files

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MogileFS – OMG Files

Tracker

Tracker

MySQL

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MogileFS – OMG Files

• Replication of file ‘classes’ happens transparently

• Storage nodes are not mirrored – replication is piecemeal

• Reading and writing go through trackers, but are performed directly upon storage nodes

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Flickr File System

• Developed by Flickr

• Proprietary

• Designed for very large scalable web app storage

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Flickr File System

• No metadata store– Deal with it yourself

• Multiple ‘StorageMaster’ nodes

• Multiple storage nodes with virtual volumes

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Flickr File System

SM

SM

SM

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Flickr File System

• Metadata stored by app– Just a virtual volume number– App chooses a path

• Virtual nodes are mirrored– Locally and remotely

• Reading is done directly from nodes

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Flickr File System

• StorageMaster nodes only used for write operations

• Reading and writing can scale separately

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Serving

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Serving files

Serving files is easy!

ApacheDisk

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Serving files

Scaling is harder

ApacheDisk

ApacheDisk

ApacheDisk

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Serving files

• This doesn’t scale well

• Primary storage is expensive– And takes a lot of space

• In many systems, we only access a small number of files most of the time

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Caching

• Insert caches between the storage and serving nodes

• Cache frequently accessed content to reduce reads on the storage nodes

• Software (Squid, mod_cache)

• Hardware (Netcache, Cacheflow)

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Why it works

• Keep a smaller working set

• Use faster hardware– Lots of RAM– SCSI– Outer edge of disks (ZCAV)

• Use more duplicates– Cheaper, since they’re smaller

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Two models

• Layer 4– ‘Simple’ balanced cache– Objects in multiple caches– Good for few objects requested many times

• Layer 7– URL balances cache– Objects in a single cache– Good for many objects requested a few times

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Replacement policies

• LRU – Least recently used

• GDSF – Greedy dual size frequency

• LFUDA – Least frequently used with dynamic aging

• All have advantages and disadvantages

• Performance varies greatly with each

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Cache Churn

• How long do objects typically stay in cache?

• If it gets too short, we’re doing badly– But it depends on your traffic profile

• Make the cached object store larger

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Problems

• Caching has some problems:

– Invalidation is hard– Replacement is dumb (even LFUDA)

• Avoiding caching makes your life (somewhat) easier

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CDN – Content Delivery Network

• Akamai, Savvis, Mirror Image Internet, etc

• Caches operated by other people– Already in-place– In lots of places

• GSLB/DNS balancing

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Edge networks

Origin

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Edge networks

Origin

Cache

Cache

Cache

CacheCache

Cache

CacheCache

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CDN Models

• Simple model– You push content to them, they serve it

• Reverse proxy model– You publish content on an origin, they proxy

and cache it

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CDN Invalidation

• You don’t control the caches– Just like those awful ISP ones

• Once something is cached by a CDN, assume it can never change– Nothing can be deleted– Nothing can be modified

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Versioning

• When you start to cache things, you need to care about versioning

– Invalidation & Expiry– Naming & Sync

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Cache Invalidation

• If you control the caches, invalidation is possible

• But remember ISP and client caches

• Remove deleted content explicitly– Avoid users finding old content– Save cache space

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Cache versioning

• Simple rule of thumb:– If an item is modified, change its name (URL)

• This can be independent of the file system!

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Virtual versioning

• Database indicates version 3 of file

• Web app writes version number into URL

• Request comes through cache and is cached with the versioned URL

• mod_rewrite converts versioned URL to path

Version 3

example.com/foo_3.jpg

Cached: foo_3.jpg

foo_3.jpg -> foo.jpg

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Authentication

• Authentication inline layer– Apache / perlbal

• Authentication sideline– ICP (CARP/HTCP)

• Authentication by URL– FlickrFS

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Auth layer

• Authenticator sits between client and storage

• Typically built into the cache software

Cache

Authenticator

Origin

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Auth sideline

• Authenticator sits beside the cache

• Lightweight protocol used for authenticator

Cache

Authenticator

Origin

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Auth by URL

• Someone else performs authentication and gives URLs to client (typically the web app)

• URLs hold the ‘keys’ for accessing files

Cache OriginWeb Server

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BCP

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Business Continuity Planning

• How can I deal with the unexpected?– The core of BCP

• Redundancy

• Replication

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Reality

• On a long enough timescale, anything that can fail, will fail

• Of course, everything can fail

• True reliability comes only through redundancy

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Reality

• Define your own SLAs

• How long can you afford to be down?

• How manual is the recovery process?

• How far can you roll back?

• How many node x boxes can fail at once?

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Failure scenarios

• Disk failure

• Storage array failure

• Storage head failure

• Fabric failure

• Metadata node failure

• Power outage

• Routing outage

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Reliable by design

• RAID avoids disk failures, but not head or fabric failures

• Duplicated nodes avoid host and fabric failures, but not routing or power failures

• Dual-colo avoids routing and power failures, but my need duplication too

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Tend to all points in the stack

• Going dual-colo: great

• Taking a whole colo offline because of a single failed disk: bad

• We need a combination of these

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Recovery times

• BCP is not just about continuing when things fail

• How can we restore after they come back?

• Host and colo level syncing– replication queuing

• Host and colo level rebuilding

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Reliable Reads & Writes

• Reliable reads are easy– 2 or more copies of files

• Reliable writes are harder– Write 2 copies at once– But what do we do when we can’t write to

one?

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Dual writes

• Queue up data to be written– Where?– Needs itself to be reliable

• Queue up journal of changes– And then read data from the disk whose write

succeeded

• Duplicate whole volume after failure– Slow!

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Cost

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Judging cost

• Per GB?

• Per GB upfront and per year

• Not as simple as you’d hope– How about an example

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Hardware costs

Cost of hardware

Usable GB

Single Cost

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Power costs

Cost of power per year

Usable GB

Recurring Cost

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Power costs

Power installation cost

Usable GB

Single Cost

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Space costs

Cost per U

Usable GB

[ ]U’s needed (inc network)x

Recurring Cost

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Network costs

Cost of network gear

Usable GB

Single Cost

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Misc costs

Support contracts + spare disks

Usable GB

+ bus adaptors + cables[ ]Single & Recurring Costs

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Human costs

Admin cost per node

Node countx

Recurring Cost

Usable GB

[ ]

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TCO

• Total cost of ownership in two parts– Upfront– Ongoing

• Architecture plays a huge part in costing– Don’t get tied to hardware– Allow heterogeneity– Move with the market

(fin)

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Photo credits

• flickr.com/photos/ebright/260823954/• flickr.com/photos/thomashawk/243477905/• flickr.com/photos/tom-carden/116315962/• flickr.com/photos/sillydog/287354869/• flickr.com/photos/foreversouls/131972916/• flickr.com/photos/julianb/324897/• flickr.com/photos/primejunta/140957047/• flickr.com/photos/whatknot/28973703/• flickr.com/photos/dcjohn/85504455/

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You can find these slides online:

iamcal.com/talks/