And this my friends, is why I love having extra memory!
[terrypoulin@zeta ~]$ dd if=/dev/zero of=./dd.test bs=1M count=2000 2000+0 records in 2000+0 records out 2097152000 bytes (2.1 GB, 2.0 GiB) copied, 0.38554 s, 5.4 GB/s [terrypoulin@zeta ~]$ dd if=/dev/zero of=./dd.test2 bs=1M count=2000 oflag=direct 2000+0 records in 2000+0 records out 2097152000 bytes (2.1 GB, 2.0 GiB) copied, 4.01215 s, 523 MB/s [terrypoulin@zeta ~]
This machine has a cheapo 1 TB Inland Professional 2.5″ SATA SSD to serve as its system disk. But she’s got 64 freaking gigs of RAM. Yes, that’s right – sixty-four freaking gigs!
[terrypoulin@zeta ~]$ free -h
total used free shared buff/cache available
Mem: 61Gi 966Mi 57Gi 8.0Mi 4.1Gi 60Gi
Swap: 31Gi 0B 31Gi
The first dd command writes 2 GB of zeros to a file one MB at a time, as fast as the system can go. Thanks to the OS being able to say, “Hey, I’ve got memory to buffer that; carry on wayward son,” it is completed Really Damn Fast. This buffering isn’t good for the case of a slow removable disk (or IMHO, oh shit, batteries), but is very effective when doing a lot of file I/O such as compiling software or working on large projects with many files. By contrast if the system had little available memory, it wouldn’t go so fast.
The second command, effectively says the same thing but uses Direct I/O to ensure the data is spewed the disk quickly and immediately, meaning that we get the speed a decent SATA SSD can achieve when combined with its own little bit of internal buffering. But we don’t experience the crazy speeeeeeeed that is RAM.
Why is this important for Zeta? Well, Zeta is replacing Cream — which means she has an 8 TB storage array to take care of, spends most of her life dealing with networked file transfers, streaming media to client machines, and unlike Cream, will end up running several virtual machines thanks to having enough extra memory. Did I mention, fire sales on DDR4 have already begun to make way for DDR5? 😁