Andrei Popescu wrote: > Klaus Jantzen wrote: > > for (each?) partition I define there is an entry for "Reserved > > Blocks" with a suggested value of 5%. > > > > Questions: > > 1) What are these reserved blocks for? > > 2) Do I have to have them? > > 3) What are the consequences of setting the quota to 0%? > > Those are blocks reserved for the root "user". In case another user > (human or computer) fills the partition root still has to spare 5% to > potentially rescue the system which could otherwise become completely > unusable.
Yes. All correct. Additionally the reserved space is there for efficiency. The filesystem will try to allocate blocks in an efficient manor. Unix like filesystems generally do not need a separate defragmentation operation because they try to be as efficient as possible all of the time. In order to allocate blocks efficiently they need free space available. If you always operate a filesystem full capacity then it will not be possible to allocate blocks efficiently and performance will degrade significantly. Fragmentation will occur. Some percentage of the disk blocks must be kept free in order to allow the filesystem to operate at peak performance and avoid fragmentation. I don't have a reference but hopefully someone will suggest one to benchmark data. That would be great. In the beginning the reserved value was 10%. Then as disks became much larger the recommended value was reduced to 5%. Having some reserve space enforces the ability for root to take corrective action against an errant user process and also ensures that the system will always be able to perform at a reasonable level of performance. Of course the reserved space for performance requirement is based upon spinning disk media. Newer SSDs do not have such a performance limitation due to fragmentation. But they do still require space for root processes to operate in spite of an errant user process. Bob
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