How does Linux really handle writes?
Posted by jpluimers on 2026/09/14
Note that actual writes to SSD can be just as slow as to HDD. [Wayback/Archive] How does Linux really handle writes? really well describes writes to HDD which equally holds for other operating systems (like Windows) with the exception that the SYNC systems calls are named differently.
From the article:
Here’s how the write really works.
- you
echo "foo" > bar.txt- the operating system copies “foo” into a special place in memory called the page cache
- (microseconds later) the operating system tells you the write succeeded
- (asynchronously, up to 30 seconds later) the operating system actually writes “foo” to disk
If your mental model was “files live on disk”, the above is shocking. I mean, I used to think that the disk write happened immediately, but actually it can happen 30 seconds later!
Via (hopefully I can archive this link later)
Later: [Wayback/Archive] Julia Evans on Twitter: “fun read: “How does Linux really handle writes?””
This was after [Wayback/Archive] Jeroen Wiert Pluimers @wiert@mastodon.social on Twitter: “@robert_ilbrink @jsrailton @waybackmachine @archiveis Great news: Twitter has enabled viewing tweets anonymously again which means archiving tools now can archiving tweets again too. Anonymously viewing Twitter profiles with their underlying tweets however is still a no-no. 1/”
–jeroen
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