local fiber = require('fiber')
box.cfg{}
local x = box.schema.create_space('x', {format = {
    {'id', 'unsigned'},
    {'ref', 'string'},
}})
x:create_index('pk')
x:create_index('sk', {parts = {'ref'}})
local a = string.rep('a', 512 * 1024)
x:insert({1, a})
local y = box.schema.create_space('y', {format = {
    {'id', 'unsigned'},
    {'ref', 'string', foreign_key = {space = 'x', field = 'ref'}},
}})
y:create_index('pk')
y:create_index('sk', {parts = {'ref'}})
y:insert({1, a})
local b = string.rep('b', 512 * 1024)
while true do
    pcall(x.replace, x, {1, b})
    fiber.yield()
end