Hey Eli, From past experience, static, manual namespace partitioning can really get you in trouble - you have to manually keep things balanced.
The following things can go wrong: 1) One of your pesky users grows unexpectedly by a factor of 10. 2) Your entire system grew so much that there's not enough excess capacity to split and balance the cluster into new pieces - the extra bandwidth required would drive down production performance too much (or you need downtime to do it and can't afford the downtime). 3) Your production system began as a proof of concept, and your file name system makes it hard to split in a sane manner because you never planned on splitting the proof of concept in the first place! Any one of these can be solved with enough effort, but it can require a huge amount of effort if you don't realize things soon enough! In fact, I seem to remember a ACM Queue article with the original Google authors who cited explosive application growth as one reason that manual balancing quickly fell out of favor. I wouldn't deny that symlinks are an incredible tool to fight namespace growth - but it's not a 100% solution. That said, I'm looking forward to symlinks to solve a few local problems! Brian On Mar 1, 2010, at 8:15 PM, Eli Collins wrote: > On Mon, Mar 1, 2010 at 5:42 PM, Ketan Dixit <ketan.di...@gmail.com> wrote: >> Hello, >> Thank you Konstantin and Allen for your reply. The information >> provided really helped to improve my understanding. >> However I still have few questions. >> How Symlinks/ soft links are used to solve the probem of partitioning. >> (Where do the symlinks point to? All the mapping is >> stored in memory but symlinks point to file objects? This is little >> confusing to me) >> Can you please provide insight into this? > > The idea is to use symlinks to present a single namespace to clients > that is backed by multiple file systems (hdfs or other supported > hadoop file systems). Eg a "root" HDFS file system could contain links > to other file systems, eg /dir1 could point to S3, /dir2 could point > to a local file system, /dir3 could point to another HDFS file system, > etc. Clients always contact the "root" HDFS file system but are > transparently redirected to other file systems by symlinks. This way a > single namespace is partitioned across multiple file systems, but the > client only needs to know about the root file system. This > partitioning is static (you have to establish the symlinks), though > you can grow on the fly by adding file systems and links that point to > them. > > Thanks, > Eli
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